Selenium preloaded cathode for alkali metal-selenium secondary battery and production process

ABSTRACT

A method of producing a pre-selenized (selenium-preloaded) active cathode layer for a rechargeable alkali metal-selenium cell; the method comprising: (a) Preparing an integral layer of porous graphitic structure having a specific surface area greater than 100 m 2 /g; (b) Preparing an electrolyte comprising a solvent and a selenium source; (c) Preparing an anode; and (d) Bringing the integral layer and the anode in ionic contact with the electrolyte and imposing an electric current between the anode and the integral layer (serving as a cathode) to electrochemically deposit nanoscaled selenium particles or coating on the graphene surfaces. The selenium particles or coating have a thickness or diameter smaller than 20 nm (preferably &lt;10 nm, more preferably &lt;5 nm or even &lt;3 nm) and occupy a weight fraction of at least 70% (preferably &gt;90% or even &gt;95%).

FIELD OF THE INVENTION

The present invention provides a unique cathode composition andstructure in a secondary or rechargeable alkali metal-selenium battery,including the lithium-selenium battery and sodium-selenium battery, anda process for producing same.

BACKGROUND

Rechargeable lithium-ion (Li-ion) and lithium metal batteries (includingLi-sulfur and Li metal-air batteries) are considered promising powersources for electric vehicle (EV), hybrid electric vehicle (HEV), andportable electronic devices, such as lap-top computers and mobilephones. Lithium as a metal element has the highest capacity (3,861mAh/g) compared to any other metal or metal-intercalated compound as ananode active material (except Li_(4.4)Si, which has a specific capacityof 4,200 mAh/g). Hence, in general, Li metal batteries have asignificantly higher energy density than lithium ion batteries.

Historically, rechargeable lithium metal batteries were produced usingnon-lithiated compounds having relatively high specific capacities, suchas TiS₂, MoS₂, MnO₂, COO₂, and V₂O₅, as the cathode active materials,which were coupled with a lithium metal anode. When the battery wasdischarged, lithium ions were transferred from the lithium metal anodethrough the electrolyte to the cathode, and the cathode becamelithiated. Unfortunately, upon repeated charges/discharges, the lithiummetal resulted in the formation of dendrites at the anode thatultimately grew to penetrate through the separator, causing internalshorting and explosion. As a result of a series of accidents associatedwith this problem, the production of these types of secondary batterieswas stopped in the early 1990's, giving ways to lithium-ion batteries.

In lithium-ion batteries, pure lithium metal sheet or film was replacedby carbonaceous materials as the anode. The carbonaceous materialabsorbs lithium (through intercalation of lithium ions or atoms betweengraphene planes, for instance) and desorbs lithium ions during there-charge and discharge phases, respectively, of the lithium ion batteryoperation. The carbonaceous material may comprise primarily graphitethat can be intercalated with lithium and the resulting graphiteintercalation compound may be expressed as Li_(x)C₆, where x istypically less than 1.

Although lithium-ion (Li-ion) batteries are promising energy storagedevices for electric drive vehicles, state-of-the-art Li-ion batterieshave yet to meet the cost and performance targets. Li-ion cellstypically use a lithium transition-metal oxide or phosphate as apositive electrode (cathode) that de/re-intercalates Li⁺ at a highpotential with respect to the carbon negative electrode (anode). Thespecific capacity of lithium transition-metal oxide or phosphate basedcathode active material is typically in the range from 140-180 mAh/g. Asa result, the specific energy of commercially available Li-ion cells istypically in the range from 120-240 Wh/kg, most. These specific energyvalues are two to three times lower than what would be required forbattery-powered electric vehicles to be widely accepted.

With the rapid development of hybrid (HEV), plug-in hybrid electricvehicles (HEV), and all-battery electric vehicles (EV), there is anurgent need for anode and cathode materials that provide a rechargeablebattery with a significantly higher specific energy, higher energydensity, higher rate capability, long cycle life, and safety. Two of themost promising energy storage devices are the lithium-sulfur (Li—S) celland lithium-selenium (Li—Se) cell since the theoretical capacity of Liis 3,861 mAh/g, that of S is 1,675 mAh/g, and that of Se is 675 mAh/g.Compared with conventional intercalation-based Li-ion batteries, Li—Sand Li—Se cells have the opportunity to provide a significantly higherenergy density (a product of capacity and voltage). With a significantlyhigher electronic conductivity, Se is a more effective cathode activematerial and, as such, Li—Se potentially can exhibit a higher ratecapability.

However, Li—Se cell is plagued with several major technical problemsthat have hindered its widespread commercialization:

-   (1) All prior art Li—Se cells have dendrite formation and related    internal shorting issues;-   (2) The cell tends to exhibit significant capacity decay during    discharge-charge cycling. This is mainly due to the high solubility    of selenium and lithium poly selenide anions formed as reaction    intermediates during both discharge and charge processes in the    polar organic solvents used in electrolytes. During cycling, the    anions can migrate through the separator to the Li negative    electrode whereupon they are reduced to solid precipitates, causing    active mass loss. In addition, the solid product that precipitates    on the surface of the positive electrode during discharge becomes    electrochemically irreversible, which also contributes to active    mass loss. This phenomenon is commonly referred to as the Shuttle    Effect. This process leads to several problems: high self-discharge    rates, loss of cathode capacity, corrosion of current collectors and    electrical leads leading to loss of electrical contact to active    cell components, fouling of the anode surface giving rise to    malfunction of the anode, and clogging of the pores in the cell    membrane separator which leads to loss of ion transport and large    increases in internal resistance in the cell.-   (3) Presumably, nanostructured mesoporous carbon materials could be    used to hold the Se or lithium polyselenide in their pores,    preventing large out-flux of these species from the porous carbon    structure through the electrolyte into the anode. However, the    fabrication of the proposed highly ordered mesoporous carbon    structure requires a tedious and expensive template-assisted    process. It is also challenging to load a large proportion of    selenium into these mesoscaled pores using a physical vapor    deposition or solution precipitation process. Typically the maximum    loading of Se in these porous carbon structures is less than 50%.

Despite the various approaches proposed for the fabrication of highenergy density Li—Se cells, there remains a need for cathode materials,production processes, and cell operation methods that retard theout-diffusion of Se or lithium polyselenide from the cathodecompartments into other components in these cells, improve theutilization of electro-active cathode materials (Se utilizationefficiency), and provide rechargeable Li—Se cells with high capacitiesover a large number of cycles.

Most significantly, lithium metal (including pure lithium, lithiumalloys of high lithium content with other metal elements, orlithium-containing compounds with a high lithium content; e.g. >80% orpreferably >90% by weight Li) still provides the highest anode specificcapacity as compared to essentially all other anode active materials(except pure silicon, but silicon has pulverization issues). Lithiummetal would be an ideal anode material in a lithium-selenium secondarybattery if dendrite related issues could be addressed.

Sodium metal (Na) and potassium metal (K) have similar chemicalcharacteristics to Li and the selenium cathode in sodium-selenium cells(Na—Se batteries) or potassium-selenium cells (K—Se) face the sameissues observed in Li—S batteries, such as: (i) low active materialutilization rate, (ii) poor cycle life, and (iii) low Coulumbicefficiency. Again, these drawbacks arise mainly from insulating natureof Se, dissolution of polyselenide intermediates in liquid electrolytes(and related Shuttle effect), and large volume change duringcharge/discharge.

Hence, an object of the present invention is to provide a rechargeableLi—Se battery that exhibits an exceptionally high specific energy orhigh energy density. One particular technical goal of the presentinvention is to provide a Li metal-selenium or Li ion-selenium cell witha cell specific energy greater than 300 Wh/kg, preferably greater than350 Wh/kg, and more preferably greater than 400 Wh/kg (all based on thetotal cell weight).

It may be noted that in most of the open literature reports (scientificpapers) and patent documents, scientists or inventors choose to expressthe cathode specific capacity based on the selenium or lithiumpolyselenide weight alone (not the total cathode composite weight), butunfortunately a large proportion of non-active materials (those notcapable of storing lithium, such as conductive additive and binder) istypically used in their Li—Se cells. For practical use purposes, it ismore meaningful to use the cathode composite weight-based capacityvalue.

A specific object of the present invention is to provide a rechargeablelithium-selenium cell based on rational materials and battery designsthat overcome or significantly reduce the following issues commonlyassociated with conventional Li—Se cells: (a) dendrite formation(internal shorting); (b) low electric and ionic conductivities ofselenium, requiring large proportion (typically 30-55%) of non-activeconductive fillers and having significant proportion of non-accessibleor non-reachable selenium or lithium polyselenide); (c) dissolution oflithium polyselenide in electrolyte and migration of dissolved lithiumpolyselenide from the cathode to the anode (which irreversibly reactwith lithium at the anode), resulting in active material loss andcapacity decay (the shuttle effect); and (d) short cycle life.

In addition to overcoming the aforementioned problems, another object ofthe present invention is to provide a simple, cost-effective, andeasy-to-implement approach to preventing potential Li metaldendrite-induced internal short circuit and thermal runaway problems inLi metal-selenide batteries.

SUMMARY OF THE INVENTION

The present invention provides a unique cathode composition andstructure in a secondary or rechargeable alkali metal-selenium battery,including the lithium-selenium battery and sodium-selenium battery. Thelithium-selenium battery can include the lithium metal-selenium battery(having lithium metal as the anode active material and selenium as thecathode active material) and the lithium ion-selenium battery (e.g. Sior graphite as the anode active material and selenium as the cathodeactive material). The sodium-selenium battery can include the sodiummetal-selenium battery (having sodium metal as the anode active materialand selenium as the cathode active material) and the sodium ion-seleniumbattery (e.g. hard carbon as the anode active material and selenium asthe cathode active material).

The present invention provides an electrochemical method of producing apre-selenized active cathode layer for use in a rechargeable alkalimetal-selenium cell. The term “pre-selenized” means pre-loading seleniuminto a cathode active material before this cathode active material isincorporated into a battery cell (if this pre-selenization procedure isconducted outside of the intended lithium-selenium cell) or before thebattery cell is operated to provide power to an external device (if thispre-selenization procedure is conducted in situ inside the intendedlithium-selenium cell during the first charge cycle).

Such an electrochemical method is surprisingly capable of uniformlydepositing an ultra-thin selenium (Se) coating layer or ultra-smallsmall Se particles (<20 nm, more preferably and typically <10 nm, mosttypically and preferably <5 nm, or even <3 nm) on massive graphenesurfaces, yet achieving a large proportion of Se (the cathode activematerial) relative to the supporting substrate (graphene or exfoliatedgraphite materials). These electrochemically deposited Se coating orparticles remain well-adhered to the graphene surfaces during repeatedcharges/discharges, enabling an unusually high long cycle life. Theultra-thin dimensions also enable high storing/releasing rates of alkalimetal ions (Li⁺, Na⁺, and/or K⁺) and, hence, exceptional rate capabilityor power density.

For the purpose of describing the preferred embodiments of the instantinvention, Li ions, Li metal, and Li—Se cells are used as examples. But,the same or similar procedures are applicable to other alkali metals andalkali metal-selenium cells (e.g. Na—Se cells and K—Se cells) Thismethod comprises the following four elements, (a)-(d):

-   -   a) Preparing an integral layer of porous graphene or graphitic        structure having massive graphene surfaces with a specific        surface area greater than 100 m²/g. The porous graphitic        structure has a specific surface area preferably >500 m²/g and        more preferably >700 m²/g, and most preferably >1,000 m²/g.        -   The layer of porous graphitic structure contains a graphene            material selected from pristine graphene, graphene oxide,            reduced graphene oxide, graphene fluoride, graphene            chloride, graphene bromide, graphene iodide, hydrogenated            graphene, nitrogenated graphene, boron-doped graphene,            nitrogen-doped graphene, chemically functionalized graphene,            or a combination thereof. Alternatively or additionally, the            graphene structure contains an exfoliated graphite material            selected from exfoliated graphite worms, expanded graphite            flakes, or recompressed graphite worms or flakes. The            graphene structure comprises multiple sheets or flakes of a            graphene material or exfoliated graphite material that are            intersected or interconnected to form the integral layer            with or without a binder to bond the multiple sheets            together, and with or without a conductive filler being            included in the integral layer. It is surprising to discover            that multiple graphene sheets can be packed together to form            an electrode layer of high structural integrity without            utilizing a binder resin (effectively reducing the            proportion of non-active materials in the cathode). These            multiple sheets or flakes, along with the optional            ingredients (binder, conductive additives, etc.), are            combined to form a porous graphitic structure that must            still have a specific surface area greater than 100 m²/g            (preferably >500 m²/g and more preferably >700 m²/g, and            most preferably >1,000 m²/g).        -   The layer of porous graphitic structure contains 0-49%            (preferably 0-30%, more preferably 0-20%, and further            preferably 0-10%) by weight of selenium or            selenium-containing compound pre-loaded therein prior to the            current electrochemical deposition), based on the total            weights of all ingredients in the layer. Although not            preferred, one can pre-load 0.01% to 49% of S on graphene            surfaces.    -   b) Preparing an electrolyte comprising a solvent (preferably        organic solvent and/or ionic liquid) and a selenium source        dissolved or dispersed in the solvent;    -   c) Preparing an anode (this anode layer can be an anode active        material layer in an intended Li—Se cell or an electrode in an        external chamber/reactor that is external or unrelated to the        intended Li—Se cell); and    -   d) Bringing the integral layer of porous graphitic structure and        the anode in ionic contact with the electrolyte (e.g. immersing        all these components in a chamber or reactor being external to        the intended Li—Se cell, or encasing these three components        inside the Li—Se cell) and imposing an electric current between        the anode and the integral layer of porous graphitic structure        (serving as a cathode) with a sufficient current density for a        sufficient period of time to electrochemically deposit        nanoscaled selenium particles or coating on the graphene        surfaces to form the pre-selenized active cathode layer, wherein        the particles or coating have a thickness or diameter smaller        than 20 nm (preferably <10 nm, more preferably <5 nm, and        further preferably <3 nm) and wherein the nanoscaled selenium        particles or coating occupy a weight fraction of at least 70%        (preferably >80%, more preferably >90%, and most        preferably >95%) based on the total weights of the selenium        particles or coating and the graphene material combined.        When these three components (porous graphitic structure, anode,        and electrolyte) are encased inside the Li—Se cell, nano        selenium is electrochemically deposited in situ in the cathode        inside the battery cell. When the three components are        implemented in an external container (chamber or reactor outside        of the intended Li—S battery cell), nano selenium is deposited        onto graphene surfaces through the “external electrochemical        deposition” route.

In one preferred embodiment, the selenium source is selected fromM_(x)Se_(y), wherein x is an integer from 1 to 3 and y is an integerfrom 1 to 10, and M is a metal element selected from an alkali metal, analkaline metal selected from Mg or Ca, a transition metal, a metal fromgroups 13 to 17 of the periodic table, or a combination thereof. In adesired embodiment, the metal element M is selected from Li, Na, K, Mg,Zn, Cu, Ti, Ni, Co, Fe, or Al. In a particularly desired embodiment,M_(x)Se_(y) is selected from Li₂Se₆, Li₂Se₇, Li₂Se₈, Li₂Se₉, Li₂Se₁₀,Na₂Se₆, Na₂Se₇, Na₂Se₈, Na₂Se₉, Na₂Se₁₀, K₂Se₆, K₂Se₇, K₂Se₈, K₂Se₉,K₂Se₁₀, or a combination thereof.

In one embodiment, the anode comprises an anode active material selectedfrom an alkali metal, an alkaline metal, a transition metal, a metalfrom groups 13 to 17 of the periodic table, or a combination thereof.

In one embodiment, the method further comprises a procedure ofdepositing an element Z to the porous graphitic structure whereinelement Z is mixed with selenium or formed as discrete Z coating orparticles having a dimension less than 100 nm (preferably <20 nm,further preferably <10 nm, even more preferably <5 nm, and mostpreferably <3 nm) and Z element is selected from Sn, Sb, Bi, S, and/orTe. The procedure of depositing element Z may be preferably selectedfrom electrochemical deposition, chemical deposition, or solutiondeposition. We have discovered that the addition of some amount (lessthan 50%, preferably less than 20% by weight) of Sn, Sb, Bi, S, or Tecan lead to improved cathode conductivity and/or higher specificcapacity.

The electrolyte may further comprise a metal salt selected from lithiumperchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithiumborofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluoro-metasulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimidelithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-Fluoroalkyl-Phosphates(LiPF₃(CF₂CF₃)₃), lithium bisperfluoro-ethysulfonylimide (LiBETI),lithium bis(trifluoromethanesulphonyl)imide, lithiumbis(fluorosulphonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI),an ionic liquid-based lithium salt, sodium perchlorate (NaClO₄),potassium perchlorate (KClO₄), sodium hexafluorophosphate (NaPF₆),potassium hexafluorophosphate (KPF₆), sodium borofluoride (NaBF₄),potassium borofluoride (KBF₄), sodium hexafluoroarsenide, potassiumhexafluoroarsenide, sodium trifluoro-metasulfonate (NaCF₃SO₃), potassiumtrifluoro-metasulfonate (KCF₃SO₃), bis-trifluoromethyl sulfonylimidesodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide (NaTFSI),bis-trifluoromethyl sulfonylimide potassium (KN(CF₃SO₂)₂), or acombination thereof.

The solvent may be selected from 1,3-dioxolane (DOL),1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME),poly(ethylene glycol) dimethyl ether (PEGDME), diethylene glycol dibutylether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, ethylenecarbonate (EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC),diethyl carbonate (DEC), ethyl propionate, methyl propionate, propylenecarbonate (PC), gamma-butyrolactone (γ-BL), acetonitrile (AN), ethylacetate (EA), propyl formate (PF), methyl formate (MF), toluene, xylene,methyl acetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate(VC), allyl ethyl carbonate (AEC), a hydrofluoroether, a roomtemperature ionic liquid solvent, or a combination thereof.

In one preferred embodiment, the electrochemical deposition is conductedbefore the cathode active layer is incorporated into an intendedlithium-selenium (Li—Se) battery cell, Na—Se cell, or K—Se cell. Inother words, the anode, the electrolyte, and the integral layer ofporous graphitic structure (serving as a cathode layer) are disposed inan external container outside of a lithium-selenium cell. The neededapparatus is similar to an electro-plating system. The step ofelectrochemically depositing nanoscaled selenium particles or coating onthe graphene surfaces is conducted outside the lithium-selenium cell andprior to the battery cell fabrication.

In another embodiment, the anode, the electrolyte, and the integrallayer of porous graphitic structure are included inside an alkalimetal-selenium cell (e.g. lithium-selenium cell). In other words, thebattery cell itself is an electrochemical deposition system forpre-selenization of the cathode and the step of electrochemicallydepositing nanoscaled selenium particles or coating on the graphenesurfaces occurs after the lithium-selenium cell is fabricated. Thiselectrochemical deposition procedure is conducted during the firstcharge cycle of the Li—Se cell.

A special and highly advantageous feature of the inventive method is thenotion that this method enables the selenium to be deposited in a thincoating or ultra-fine particle form (thus, providing ultra-short lithiumion diffusion paths and, hence, ultra-fast reaction times for fastbattery charges and discharges) while achieving a relatively highproportion of selenium (the active material responsible for storinglithium) and, thus, high specific lithium storage capacity of theresulting cathode active layer in terms of high mAh/g (based on thetotal weight of the cathode layer, including the masses of the activematerial, Se, supporting graphene sheets, binder resin, and conductivefiller combined). It is of significance to note that one might be ableto use a prior art procedure to deposit small Se particles, but cannotachieve a high Se proportion at the same time, or to achieve a highproportion of Se, but only in large particles or thick film form. Theprior art procedures have not been able to achieve both at the sametime.

This is why it is such an unexpected and highly advantageous thing toachieve a high selenium loading and yet, concurrently, form anultra-thin coating or ultra-small diameter particles of selenium. Thishas not been possible with any prior art selenium loading techniques.For instance, we have been able to deposit nanoscaled selenium particlesor coating that occupy a >90% weight fraction of the cathode layer andyet maintain a coating thickness or particle diameter<3 nm. This isquite a feat in the art of lithium-selenium batteries. In anotherexample, we have achieved a >95% Se loading at an average Se coatingthickness of 4.5-7.1 nm. These ultra-thin dimensions (3-7 nm) enablefacile cathode reactions and nearly perfect selenium utilizationefficiency, something that no prior worker has been able to achieve.

The present invention also provides a pre-selenized active cathode layerproduced by the above-described method and a rechargeable alkalimetal-selenium cell (e.g. lithium-selenium battery cell) that containssuch a cathode layer. Preferably, the graphene sheets in the integrallayer of porous graphitic structure are chemically bonded together withan adhesive resin. Typically, such a rechargeable alkali metal-seleniumcell comprises an anode active material layer, an optional anode currentcollector, a porous separator and/or an electrolyte, a pre-selenizedactive cathode layer, and an optional cathode current collector.

In the invented rechargeable alkali metal-selenium cell, the electrolytemay be selected from polymer electrolyte, polymer gel electrolyte,composite electrolyte, ionic liquid electrolyte, non-aqueous liquidelectrolyte, soft matter phase electrolyte, solid-state electrolyte, ora combination thereof. The electrolyte preferably contains an alkalimetal salt (lithium salt, sodium salt, and/or potassium salt) selectedfrom lithium perchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆),lithium borofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆),lithium trifluoro-metasulfonate (LiCF₃SO₃), bis-trifluoromethylsulfonylimide lithium (LiN(CF₃SO₂)₂, lithium bis(oxalato)borate (LiBOB),lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphates(LiPF₃(CF₂CF₃)₃), lithium bisperfluoroethysulfonylimide (LiBETI), anionic liquid salt, sodium perchlorate (NaClO₄), potassium perchlorate(KClO₄), sodium hexafluorophosphate (NaPF₆), potassiumhexafluorophosphate (KPF₆), sodium borofluoride (NaBF₄), potassiumborofluoride (KBF₄), sodium hexafluoroarsenide, potassiumhexafluoroarsenide, sodium trifluoro-metasulfonate (NaCF₃SO₃), potassiumtrifluoro-metasulfonate (KCF₃SO₃), bis-trifluoromethyl sulfonylimidesodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide (NaTFSI),bis-trifluoromethyl sulfonylimide potassium (KN(CF₃SO₂)₂), or acombination thereof.

Ionic liquids (ILs) are a new class of purely ionic, salt-like materialsthat are liquid at unusually low temperatures. The official definitionof ILs uses the boiling point of water as a point of reference: “Ionicliquids are ionic compounds which are liquid below 100° C.”. Aparticularly useful and scientifically interesting class of ILs is theroom temperature ionic liquid (RTIL), which refers to the salts that areliquid at room temperature or below. RTILs are also referred to asorganic liquid salts or organic molten salts. An accepted definition ofan RTIL is any salt that has a melting temperature lower than ambienttemperature. Common cations of RTILs include, but not limited to,tetraalkylammonium, di-, tri-, or tetra-alkylimidazolium,alkylpyridinium, dialkylpyrrolidinium, dialkylpiperidinium,tetraalkylphosphonium, and trialkylsulfonium. Common anions of RTILsinclude, but not limited to, BF₄ ⁻, B(CN)₄ ⁻, CH₃BF₃ ⁻, CH₂CHBF₃ ⁻,CF₃BF₃ ⁻, C₂F₅BF₃ ⁻, n-C₃F₇BF₃ ⁻, n-C₄F₉BF₃ ⁻, PF₆ ⁻, CF₃CO₂ ⁻, CF₃SO₃⁻, N(SO₂CF₃)₂ ⁻, N(COCF₃)(SO₂CF₃)⁻, N(SO₂F)₂ ⁻, N(CN)₂ ⁻, C(CN)₃ ⁻,SCN⁻, SeCN⁻, CuCl₂, AlCl₄ ⁻, F(HF)_(2.3) ⁻, etc.

As examples, the solvent may be selected from ethylene carbonate (EC),dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate(DEC), ethyl propionate, methyl propionate, propylene carbonate (PC),gamma.-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA),propyl formate (PF), methyl formate (MF), toluene, xylene or methylacetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate (VC),allyl ethyl carbonate (AEC), 1,3-dioxolane (DOL), 1,2-dimethoxyethane(DME), tetraethylene glycol dimethylether (TEGDME), poly(ethyleneglycol) dimethyl ether (PEGDME), diethylene glycol dibutyl ether(DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, roomtemperature ionic liquid, or a combination thereof.

In an embodiment, the rechargeable alkali metal-selenium cell mayfurther comprise a layer of protective material disposed between theanode and the porous separator, wherein the protective material is aconductor to the intended alkali metal ions (e.g. Li⁺, Na⁺, or K⁺). In apreferred embodiment, the protective material consists of a solidelectrolyte.

In an embodiment, the anode active material layer contains an anodeactive material selected from lithium metal, sodium metal, potassiummetal, a lithium metal alloy, sodium metal alloy, potassium metal alloy,a lithium intercalation compound, a sodium intercalation compound, apotassium intercalation compound, a lithiated compound, a sodiatedcompound, a potassium-doped compound, lithiated titanium dioxide,lithium titanate, lithium manganate, a lithium transition metal oxide,Li₄Ti₅O₁₂, or a combination thereof. This anode active material layercan be optionally coated on an anode current collector (such as Cufoil).

In another embodiment, the lithium-selenium battery cell is an alkalimetal ion-selenium cell (e.g. lithium ion-selenium cell, sodium-ionselenium cell, potassium ion-selenium cell) wherein the anode activematerial layer contains an anode active material selected from the groupconsisting of: (a) silicon (Si), germanium (Ge), tin (Sn), lead (Pb),antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), nickel (Ni),cobalt (Co), manganese (Mn), titanium (Ti), iron (Fe), and cadmium (Cd),and lithiated versions thereof, (b) alloys or intermetallic compounds ofSi, Ge, Sn, Pb, Sb, Bi, Zn, Al, or Cd with other elements, and lithiatedversions thereof, wherein said alloys or compounds are stoichiometric ornon-stoichiometric; (c) oxides, carbides, nitrides, sulfides,phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al,Fe, Ni, Co, Ti, Mn, or Cd, and their mixtures or composites, andlithiated versions thereof; (d) salts and hydroxides of Sn and lithiatedversions thereof; (e) carbon or graphite materials and prelithiatedversions thereof; and combinations thereof.

We have discovered that the use of these types of anode active materials(instead of lithium metal foil, for instance) can eliminates thedendrite issue. The resulting battery cells are herein referred to aslithium ion selenium cells, a new breed of lithium-selenium cells.

Although in general not required, the cathode active layer of therechargeable alkali metal-selenium cell may contain a conductive fillerselected from the group consisting of electrospun nanofibers,vapor-grown carbon or graphite nanofibers, carbon or graphite whiskers,carbon nanotubes, carbon nanowires, expanded graphite flakes, metalnanowires, metal-coated nanowires or nanofibers, conductivepolymer-coated nanowires or nanofibers, and combinations thereof. In anembodiment, the conductive filler comprises a fiber selected from thegroup consisting of an electrically conductive electrospun polymerfiber, electrospun polymer nanocomposite fiber comprising a conductivefiller, nano carbon fiber obtained from carbonization of an electrospunpolymer fiber, electrospun pitch fiber, and combinations thereof.

In the invented rechargeable alkali metal-selenium cell, the graphenematerial preferably comprises nanographene sheets or platelets with athickness less than 10 nm, preferably <5 nm, further preferably <2 nm,and most preferably <1 nm. The nanographene sheets or platelets arepreferably selected from single-layer or few-layer pristine graphene,wherein few-layer is defined as 10 planes of hexagonal carbon atoms orless. Thinner graphene sheets (particularly single-layer or few-layersheets) make it possible to make a porous graphitic structure that has aspecific surface area greater than 500 m²/g, preferably >700 m²/g, andmore preferably >1,000 m²/g, and most preferably >1,500 m²/g. Theultra-thin sheets of various graphene materials make it possible toproduce a cathode active layer having massive surfaces to support thinSe coating deposited thereon. This in turn makes it possible to makegood or full utilization of the cathode active material (i.e. Se). Wehave achieved a cathode active material utilization rate>90% oreven >99%. In the rechargeable alkali metal-selenium cell, the cathodecontains at least 70% by weight of selenium (preferably >80% and furtherpreferably >90%) based on the total weight of said porous graphiticstructure and selenium combined

In the rechargeable alkali metal-selenium cell, the binder material (ifdesired) is selected from a resin, a conductive polymer, coal tar pitch,petroleum pitch, mesophase pitch, coke, or a derivative thereof.

In the rechargeable alkali metal-selenium cell, the cathode may furthercomprise additional selenium, selenium-containing molecule,selenium-containing compound, selenium-carbon polymer, or a combinationthereof, which is loaded before the cell is manufactured.

The presently invented cell provides a reversible specific capacity oftypically no less than 400 mAh per gram based on the total weight of theintegral cathode layer (the weights of Se, graphene material, optionalbinder, and optional conductive filler combined), not just based on theactive material weight (selenium) only. Most of the scientific papersand patent documents reported their selenium cathode specific capacitydata based on selenium weight only.

More typically and preferably, the reversible specific capacity is noless than 500 mAh per gram and often exceeds 550 or even 600 mAh pergram of entire cathode layer. The high specific capacity of thepresently invented cathode, when in combination with a lithium anode,leads to a cell specific energy of no less than 300 Wh/kg based on thetotal cell weight including anode, cathode, electrolyte, separator, andcurrent collector weights combined. This specific energy value is notbased on the cathode active material weight or cathode layer weight only(as sometimes did in open literature or patent applications); instead,this is based on entire cell weight. In many cases, the cell specificenergy is higher than 350 Wh/kg and, in some examples, exceeds 400Wh/kg.

These and other advantages and features of the present invention willbecome more transparent with the description of the following best modepractice and illustrative examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) Schematic of the commonly used procedures for producingexfoliated graphite worms and graphene sheets;

FIG. 1(B) Another schematic drawing to illustrate the process forproducing exfoliated graphite, expanded graphite flakes, and graphenesheets.

FIG. 2(A) SEM images of exfoliated graphite worms imaged at a lowmagnification;

FIG. 2(B) same graphite worm as in (A), but taken at a highermagnification;

FIG. 2(C) TEM image of single-layer graphene sheets partially stackedtogether.

FIG. 3 The charge and discharge cycling results of three Li—Se cells,one containing a presently invented cathode structure prepared by theexternal electrochemical deposition of selenium into a layer of porousrecompressed exfoliated graphite paper, the second cell containing acathode prepared by using PVD to deposit selenium in a comparable sheetof recompressed exfoliated graphite paper, and the third containing acathode material prepared by ball-milling a mixture of Se powder andactivated carbon powder.

FIG. 4 Ragone plots (cell power density vs. cell energy density) of fiveLi metal-selenium cells: pristine graphene (PG)-based cathode containingelectrochemically deposited selenium particles (87% Se), and mesoporouscarbon-based cathode containing melt infiltration-deposited seleniumparticles (64.5% Se).

FIG. 5 Ragone plots (cell power density vs. cell energy density) of 4alkali metal-selenium cells: Na—Se cell featuring a RGO-based cathodecontaining electrochemically deposited selenium particles (70% Se),Na—Se cell featuring a RGO-based cathode containing chemically depositedSe particles (70% Se), K—Se cell featuring a RGO-based cathodecontaining electrochemically deposited Se particles (70% Se), and K—Secell featuring a RGO-based cathode containing solution-deposited Separticles (70% Se).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

For convenience, the following discussion of preferred embodiments isprimarily based on cathodes for Li—Se cells, but the same or similarmethods are applicable to deposition of Se in the cathode for the Na—Seand K—Se cells. Examples are presented for Li—Se cells, Na—Se cells, andK—Se cells.

A. Alkali Metal-Selenium Cells (Using Lithium-Selenium Cells as anExample)

The specific capacity and specific energy of a Li—Se cell (or Na—Se, orK—Se cell) are dictated by the actual amount of selenium that can beimplemented in the cathode active layer (relative to other non-activeingredients, such as the binder resin and conductive filler) and theutilization rate of this selenium amount (i.e. the utilizationefficiency of the cathode active material or the actual proportion of Sthat actively participates in storing and releasing lithium ions). Ahigh-capacity and high-energy Li—Se requires a high amount of Se in thecathode active layer (i.e. relative to the amounts of non-activematerials, such as the binder resin, conductive additive, and othermodifying or supporting materials) and a high Se utilizationefficiency). The present invention provides such a cathode active layerand a method of producing such a cathode active layer, which is apre-selenized active cathode layer. This method comprises the followingfour steps, (a)-(d):

-   -   a) preparing an integral layer of porous graphitic structure        having massive graphene surfaces with a specific surface area        greater than 100 m²/g (these surfaces must be accessible to        electrolyte). The porous graphitic structure have a specific        surface area preferably >500 m²/g and more preferably >700 m²/g,        and most preferably >1,000 m²/g.    -   b) preparing an electrolyte comprising a solvent (non-aqueous        solvent, such as organic solvent and or ionic liquid) and a        selenium source dissolved or dispersed in the solvent;    -   c) preparing an anode; and    -   d) bringing the integral layer of porous graphitic structure and        the anode in ionic contact with the electrolyte (e.g. by        immersing all these components in a chamber that is external to        the intended Li—Se cell, or encasing these three components        inside the Li—Se cell) and imposing an electric current between        the anode and the integral layer of porous graphitic structure        (serving as a cathode) with a sufficient current density for a        sufficient period of time to electrochemically deposit        nanoscaled selenium particles or coating on the graphene        surfaces to form the pre-selenized active cathode layer.        The layer of porous graphitic structure recited in step (a)        contains a graphene material or an exfoliated graphite material,        wherein the graphene material is selected from pristine        graphene, graphene oxide, reduced graphene oxide, graphene        fluoride, graphene chloride, graphene bromide, graphene iodide,        hydrogenated graphene, nitrogenated graphene, boron-doped        graphene, nitrogen-doped graphene, chemically functionalized        graphene, or a combination thereof, and wherein the exfoliated        graphite material is selected from exfoliated graphite worms,        expanded graphite flakes, or recompressed graphite worms or        flakes (must still exhibit a high specific surface area, >>100        m²/g, accessible to electrolyte). The graphene structure        comprises multiple sheets of a graphene material that are        intersected or interconnected to form the integral layer with or        without a binder to bond the multiple sheets together and with        or without a conductive filler being included in the integral        layer.

It is surprising to discover that multiple graphene sheets can be packedtogether to form an electrode layer of structural integrity without theneed for a binder resin, and such a layer can hold its shape andfunctions during repeated charges and discharges of the resulting Li—Secell.

The layer of porous graphitic structure contains 0-49% (preferably0-30%, more preferably 0-30%, and further preferably 0-10%) by weight ofselenium or selenium-containing compound pre-loaded therein, based onthe weights of all ingredients in the layer prior to the step (d) ofdepositing selenium coating or particles on massive graphene sheetsurfaces. Preferably, zero (0%) selenium or selenium-containing compoundis pre-loaded into the porous graphitic structure since this pre-loadedmaterial, if not done properly, can negatively impact the subsequentpre-selenization step.

The Se particles or coating have a thickness or diameter smaller than 20nm (preferably <10 nm, more preferably <5 nm, and further preferably <3nm) and wherein the nanoscaled selenium particles or coating occupy aweight fraction of at least 70% (preferably >80%, more preferably >90%,and most preferably >95%) based on the total weights of the seleniumparticles or coating and the graphene material combined. It isadvantageous to deposit as much Se as possible yet still maintainultra-thin thickness or diameter of the Se coating or particles(e.g. >80% and <3 nm; >90% and <5 nm; and >95% and <10 nm, etc.).

B. Production of Various Graphene and Exfoliated Graphite Materials

In a preferred embodiment, the graphene electrode material is selectedfrom pristine graphene, graphene oxide, reduced graphene oxide, graphenefluoride, graphene chloride, graphene bromide, graphene iodide,hydrogenated graphene, nitrogenated graphene, chemically functionalizedgraphene, or a combination thereof. The electrode material may beselected from an exfoliated graphite material. The starting graphiticmaterial for producing any one of the above graphene or exfoliatedgraphite materials may be selected from natural graphite, artificialgraphite, mesophase carbon, mesophase pitch, mesocarbon microbead, softcarbon, hard carbon, coke, carbon fiber, carbon nanofiber, carbonnanotube, or a combination thereof.

Bulk natural graphite is a 3-D graphitic material with each graphiteparticle being composed of multiple grains (a grain being a graphitesingle crystal or crystallite) with grain boundaries (amorphous ordefect zones) demarcating neighboring graphite single crystals. Eachgrain is composed of multiple graphene planes that are oriented parallelto one another. A graphene plane in a graphite crystallite is composedof carbon atoms occupying a two-dimensional, hexagonal lattice. In agiven grain or single crystal, the graphene planes are stacked andbonded via van der Waal forces in the crystallographic c-direction(perpendicular to the graphene plane or basal plane). Although all thegraphene planes in one grain are parallel to one another, typically thegraphene planes in one grain and the graphene planes in an adjacentgrain are inclined at different orientations. In other words, theorientations of the various grains in a graphite particle typicallydiffer from one grain to another.

The constituent graphene planes of a graphite crystallite in a naturalor artificial graphite particle can be exfoliated and extracted orisolated to obtain individual graphene sheets of hexagonal carbon atoms,which are single-atom thick, provided the inter-planar van der Waalsforces can be overcome. An isolated, individual graphene plane of carbonatoms is commonly referred to as single-layer graphene. A stack ofmultiple graphene planes bonded through van der Waals forces in thethickness direction with an inter-graphene plane spacing ofapproximately 0.3354 nm is commonly referred to as a multi-layergraphene. A multi-layer graphene platelet has up to 300 layers ofgraphene planes (<100 nm in thickness), but more typically up to 30graphene planes (<10 nm in thickness), even more typically up to 20graphene planes (<7 nm in thickness), and most typically up to 10graphene planes (commonly referred to as few-layer graphene inscientific community). Single-layer graphene and multi-layer graphenesheets are collectively called “nanographene platelets” (NGPs). Graphenesheets/platelets (collectively, NGPs) are a new class of carbonnanomaterial (a 2-D nanocarbon) that is distinct from the 0-D fullerene,the 1-D CNT or CNF, and the 3-D graphite. For the purpose of definingthe claims and as is commonly understood in the art, a graphene material(isolated graphene sheets) is not (and does not include) a carbonnanotube (CNT) or a carbon nanofiber (CNF).

Our research group pioneered the development of graphene materials andrelated production processes as early as 2002: (1) B. Z. Jang and W. C.Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4,2006), application submitted on Oct. 21, 2002; (2) B. Z. Jang, et al.“Process for Producing Nano-scaled Graphene Plates,” U.S. patentapplication Ser. No. 10/858,814 (Jun. 3, 2004) (U.S. Patent Pub. No.2005/0271574); and (3) B. Z. Jang, A. Zhamu, and J. Guo, “Process forProducing Nano-scaled Platelets and Nanocomposites,” U.S. patentapplication Ser. No. 11/509,424 (Aug. 25, 2006) (U.S. Patent Pub. No.2008-0048152).

In one process, graphene materials are obtained by intercalating naturalgraphite particles with a strong acid and/or an oxidizing agent toobtain a graphite intercalation compound (GIC) or graphite oxide (GO),as illustrated in FIG. 1(A) and FIG. 1(B) (schematic drawings). Thepresence of chemical species or functional groups in the interstitialspaces between graphene planes in a GIC or GO serves to increase theinter-graphene spacing (d₀₀₂, as determined by X-ray diffraction),thereby significantly reducing the van der Waals forces that otherwisehold graphene planes together along the c-axis direction. The GIC or GOis most often produced by immersing natural graphite powder (100 in FIG.1(B)) in a mixture of seleniumic acid, nitric acid (an oxidizing agent),and another oxidizing agent (e.g. potassium permanganate or sodiumperchlorate). The resulting GIC (102) is actually some type of graphiteoxide (GO) particles if an oxidizing agent is present during theintercalation procedure. This GIC or GO is then repeatedly washed andrinsed in water to remove excess acids, resulting in a graphite oxidesuspension or dispersion, which contains discrete and visuallydiscernible graphite oxide particles dispersed in water. In order toproduce graphene materials, one can follow one of the two processingroutes after this rinsing step, briefly described below:

Route 1 involves removing water from the suspension to obtain“expandable graphite,” which is essentially a mass of dried GIC or driedgraphite oxide particles. Upon exposure of expandable graphite to atemperature in the range from typically 800-1,050° C. for approximately30 seconds to 2 minutes, the GIC undergoes a rapid volume expansion by afactor of 30-300 to form “graphite worms” (104), which are each acollection of exfoliated, but largely un-separated graphite flakes thatremain interconnected.

In Route 1A, these graphite worms (exfoliated graphite or “networks ofinterconnected/non-separated graphite flakes”) can be re-compressed toobtain flexible graphite sheets or foils (106) that typically have athickness in the range from 0.1 mm (100 μm)-0.5 mm (500 μm).Alternatively, one may choose to use a low-intensity air mill orshearing machine to simply break up the graphite worms for the purposeof producing the so-called “expanded graphite flakes” (108) whichcontain mostly graphite flakes or platelets thicker than 100 nm (hence,not a nanomaterial by definition).

In Route 1B, the exfoliated graphite is subjected to high-intensitymechanical shearing (e.g. using an ultrasonicator, high-shear mixer,high-intensity air jet mill, or high-energy ball mill) to form separatedsingle-layer and multi-layer graphene sheets (collectively called NGPs,112), as disclosed in our U.S. application Ser. No. 10/858,814 (Jun. 3,2004). Single-layer graphene can be as thin as 0.34 nm, whilemulti-layer graphene can have a thickness up to 100 nm, but moretypically less than 10 nm (commonly referred to as few-layer graphene).Multiple graphene sheets or platelets may be made into a sheet of NGPpaper using a paper-making process. This sheet of NGP paper is anexample of the porous graphitic structure layer utilized in thepresently invented process.

Route 2 entails ultrasonicating the graphite oxide suspension (e.g.graphite oxide particles dispersed in water) for the purpose ofseparating/isolating individual graphene oxide sheets from graphiteoxide particles. This is based on the notion that the inter-grapheneplane separation has been increased from 0.3354 nm in natural graphiteto 0.6-1.1 nm in highly oxidized graphite oxide, significantly weakeningthe van der Waals forces that hold neighboring planes together.Ultrasonic power can be sufficient to further separate graphene planesheets to form fully separated, isolated, or discrete graphene oxide(GO) sheets. These graphene oxide sheets can then be chemically orthermally reduced to obtain “reduced graphene oxides” (RGO) typicallyhaving an oxygen content of 0.001%-10% by weight, more typically0.01%-5% by weight, most typically and preferably less than 2% by weightof oxygen.

For the purpose of defining the claims of the instant application, NGPsor graphene materials include discrete sheets/platelets of single-layerand multi-layer (typically less than 10 layers) pristine graphene,graphene oxide, reduced graphene oxide (RGO), graphene fluoride,graphene chloride, graphene bromide, graphene iodide, hydrogenatedgraphene, nitrogenated graphene, chemically functionalized graphene,doped graphene (e.g. doped by B or N). Pristine graphene has essentially0% oxygen. RGO typically has an oxygen content of 0.001%-5% by weight.Graphene oxide (including RGO) can have 0.001%-50% by weight of oxygen.Other than pristine graphene, all the graphene materials have 0.001%-50%by weight of non-carbon elements (e.g. O, H, N, B, F, Cl, Br, I, etc.).These materials are herein referred to as non-pristine graphenematerials.

Pristine graphene, in smaller discrete graphene sheets (typically 0.3 μmto 10 μm), may be produced by direct ultrasonication (also known asliquid phase exfoliation or production) or supercritical fluidexfoliation of graphite particles. These processes are well-known in theart.

The graphene oxide (GO) may be obtained by immersing powders orfilaments of a starting graphitic material (e.g. natural graphitepowder) in an oxidizing liquid medium (e.g. a mixture of seleniumicacid, nitric acid, and potassium permanganate) in a reaction vessel at adesired temperature for a period of time (typically from 0.5 to 96hours, depending upon the nature of the starting material and the typeof oxidizing agent used). As previously described above, the resultinggraphite oxide particles may then be subjected to thermal exfoliation orultrasonic wave-induced exfoliation to produce isolated GO sheets. TheseGO sheets can then be converted into various graphene materials bysubstituting —OH groups with other chemical groups (e.g. —Br, NH₂,etc.).

Fluorinated graphene or graphene fluoride is herein used as an exampleof the halogenated graphene material group. There are two differentapproaches that have been followed to produce fluorinated graphene: (1)fluorination of pre-synthesized graphene: This approach entails treatinggraphene prepared by mechanical exfoliation or by CVD growth withfluorinating agent such as XeF₂, or F-based plasmas; (2) Exfoliation ofmultilayered graphite fluorides: Both mechanical exfoliation and liquidphase exfoliation of graphite fluoride can be readily accomplished.

Interaction of F₂ with graphite at high temperature leads to covalentgraphite fluorides (CF), or (C₂F)_(n), while at low temperaturesgraphite intercalation compounds (GIC) C_(x)F (2≤x≤24) form. In (CF)_(n)carbon atoms are sp3-hybridized and thus the fluorocarbon layers arecorrugated consisting of trans-linked cyclohexane chairs. In (C₂F)_(n)only half of the C atoms are fluorinated and every pair of the adjacentcarbon sheets are linked together by covalent C—C bonds. Systematicstudies on the fluorination reaction showed that the resulting F/C ratiois largely dependent on the fluorination temperature, the partialpressure of the fluorine in the fluorinating gas, and physicalcharacteristics of the graphite precursor, including the degree ofgraphitization, particle size, and specific surface area. In addition tofluorine (F₂), other fluorinating agents may be used, although most ofthe available literature involves fluorination with F₂ gas, sometimes inpresence of fluorides.

For exfoliating a layered precursor material to the state of individuallayers or few-layers, it is necessary to overcome the attractive forcesbetween adjacent layers and to further stabilize the layers. This may beachieved by either covalent modification of the graphene surface byfunctional groups or by non-covalent modification using specificsolvents, surfactants, polymers, or donor-acceptor aromatic molecules.The process of liquid phase exfoliation includes ultra-sonic treatmentof a graphite fluoride in a liquid medium.

The nitrogenation of graphene can be conducted by exposing a graphenematerial, such as graphene oxide, to ammonia at high temperatures(200-400° C.). Nitrogenated graphene could also be formed at lowertemperatures by a hydrothermal method; e.g. by sealing GO and ammonia inan autoclave and then increased the temperature to 150-250° C. Othermethods to synthesize nitrogen doped graphene include nitrogen plasmatreatment on graphene, arc-discharge between graphite electrodes in thepresence of ammonia, ammonolysis of graphene oxide under CVD conditions,and hydrothermal treatment of graphene oxide and urea at differenttemperatures.

The aforementioned features are further described and explained indetail as follows: As illustrated in FIG. 1(B), a graphite particle(e.g. 100) is typically composed of multiple graphite crystallites orgrains. A graphite crystallite is made up of layer planes of hexagonalnetworks of carbon atoms. These layer planes of hexagonally arrangedcarbon atoms are substantially flat and are oriented or ordered so as tobe substantially parallel and equidistant to one another in a particularcrystallite. These layers of hexagonal-structured carbon atoms, commonlyreferred to as graphene layers or basal planes, are weakly bondedtogether in their thickness direction (crystallographic c-axisdirection) by weak van der Waals forces and groups of these graphenelayers are arranged in crystallites. The graphite crystallite structureis usually characterized in terms of two axes or directions: the c-axisdirection and the a-axis (or b-axis) direction. The c-axis is thedirection perpendicular to the basal planes. The a- or b-axes are thedirections parallel to the basal planes (perpendicular to the c-axisdirection).

A highly ordered graphite particle can consist of crystallites of aconsiderable size, having a length of L_(a) along the crystallographica-axis direction, a width of L_(b) along the crystallographic b-axisdirection, and a thickness L_(c) along the crystallographic c-axisdirection. The constituent graphene planes of a crystallite are highlyaligned or oriented with respect to each other and, hence, theseanisotropic structures give rise to many properties that are highlydirectional. For instance, the thermal and electrical conductivity of acrystallite are of great magnitude along the plane directions (a- orb-axis directions), but relatively low in the perpendicular direction(c-axis). As illustrated in the upper-left portion of FIG. 1(B),different crystallites in a graphite particle are typically oriented indifferent directions and, hence, a particular property of amulti-crystallite graphite particle is the directional average value ofall the constituent crystallites.

Due to the weak van der Waals forces holding the parallel graphenelayers, natural graphite can be treated so that the spacing between thegraphene layers can be appreciably opened up so as to provide a markedexpansion in the c-axis direction, and thus form an expanded graphitestructure in which the laminar character of the carbon layers issubstantially retained. The process for manufacturing flexible graphiteis well-known in the art. In general, flakes of natural graphite (e.g.100 in FIG. 1(B)) are intercalated in an acid solution to producegraphite intercalation compounds (GICs, 102). The GICs are washed,dried, and then exfoliated by exposure to a high temperature for a shortperiod of time. This causes the flakes to expand or exfoliate in thec-axis direction of the graphite up to 80-300 times of their originaldimensions. The exfoliated graphite flakes are vermiform in appearanceand, hence, are commonly referred to as graphite worms 104. These wormsof graphite flakes which have been greatly expanded can be formedwithout the use of a binder into cohesive or integrated sheets ofexpanded graphite, e.g. webs, papers, strips, tapes, foils, mats or thelike (typically referred to as “flexible graphite” 106) having a typicaldensity of about 0.04-2.0 g/cm³ for most applications. Examples ofexfoliated graphite worms (or, simply, graphite worms) are presented inFIG. 2(A) and FIG. 2(B).

Acids, such as seleniumic acid, are not the only type of intercalatingagent (intercalant) that penetrate into spaces between graphene planesto obtain GICs. Many other types of intercalating agents, such as alkalimetals (Li, K, Na, Cs, and their alloys or eutectics), can be used tointercalate graphite to stage 1, stage 2, stage 3, etc. Stage n impliesone intercalant layer for every n graphene planes. For instance, astage-1 potassium-intercalated GIC means there is one layer of K forevery graphene plane; or, one can find one layer of K atoms insertedbetween two adjacent graphene planes in a G/K/G/K/G/KG . . . sequence,where G is a graphene plane and K is a potassium atom plane. A stage-2GIC will have a sequence of GG/K/GG/K/GG/K/GG . . . and a stage-3 GICwill have a sequence of GGG/K/GGG/K/GGG . . . , etc. These GICs can thenbe brought in contact with water or water-alcohol mixture to produceexfoliated graphite and/or separated/isolated graphene sheets.

C. Production of Integral Layer of Porous Graphitic Structure

Several techniques can be employed to fabricate a conductive layer ofporous graphitic structure (a web, mat, paper, or porous film, etc.),which is a monolithic body having desired interconnected pores that areaccessible to liquid electrolyte.

In one prior art process, the exfoliated graphite (or mass of graphiteworms) is heavily re-compressed by using a calendaring or roll-pressingtechnique to obtain flexible graphite foils (106 in FIG. 1(B)), whichare typically 100-500 m thick. This conventional flexible graphite foildoes not have a specific surface area>100 m²/g. Even though the flexiblegraphite foil is porous, most of these pores are not accessible toliquid electrolyte when immersed in an external electrochemicaldeposition chamber or incorporated in a lithium battery. For thepreparation of a desired layer of porous graphene or graphiticstructure, the compressive stress and/or the gap between rollers can bereadily adjusted to obtain a desired layer of porous graphitic structurethat has massive graphene surfaces (having a specific surface area>100m²/g) accessible to liquid electrolyte and available for receiving theselenium coating or nanoparticles deposited thereon.

Exfoliated graphite worms may be subjected to high-intensity mechanicalshearing/separation treatments using a high-intensity air jet mill,high-intensity ball mill, or ultrasonic device to produce separatednanographene platelets (NGPs) with all the graphene platelets thinnerthan 100 nm, mostly thinner than 10 nm, and, in many cases, beingsingle-layer graphene (also illustrated as 112 in FIG. 1(B)). An NGP iscomposed of a graphene sheet or a plurality of graphene sheets with eachsheet being a two-dimensional, hexagonal structure of carbon atoms. Amass of multiple NGPs (including discrete sheets/platelets ofsingle-layer and/or few-layer graphene or graphene oxide may be madeinto a graphene film/paper (114 in FIG. 1(B)) using a film- orpaper-making process.

Alternatively, with a low-intensity shearing, graphite worms tend to beseparated into the so-called expanded graphite flakes (108 in FIG. 1(B)having a thickness>100 nm. These flakes can be formed into graphitepaper or mat 106 using a paper- or mat-making process, with or without aresin binder. In one preferred embodiment of the present invention, theporous web can be made by using a slurry molding or a flake/binderspraying technique. These methods can be carried out in the followingways:

As a wet process, aqueous slurry is prepared which comprises a mixtureof graphene sheets or expanded graphite flakes and, optionally, about0.1 wt. % to about 10 wt. % resin powder binder (e.g., phenolic resin).The slurry is then directed to impinge upon a sieve or screen, allowingwater to permeate through, leaving behind sheets/flakes and the binder.As a dry process, the directed sheet/flake spray-up process utilizes anair-assisted flake/binder spraying gun, which conveys flakes/sheets andan optional binder to a molding tool (e.g., a perforated metal screenshaped identical or similar to the part to be molded). Air goes throughperforations, but the solid components stay on the molding tool surface.

Each of these routes can be implemented as a continuous process. Forinstance, the process begins with pulling a substrate (porous sheet)from a roller. The moving substrate receives a stream of slurry (asdescribed in the above-described slurry molding route) from above thesubstrate. Water sieves through the porous substrate with all otheringredients (a mixture of graphene sheets or graphite flakes, optionalconductive fillers, and an optional binder) remaining on the surface ofthe substrate being moved forward to go through a compaction stage by apair of compaction rollers. Heat may be supplied to the mixture before,during, and after compaction to help cure the thermoset binder forretaining the shape of the resulting web or mat. The web or mat, withall ingredients held in place by the thermoset binder, may be storedfirst (e.g., wrapped around a roller). Similar procedures may befollowed for the case where the mixture is delivered to the surface of amoving substrate by compressed air, like in a directed fiber/binderspraying process. Air will permeate through the porous substrate withother solid ingredients trapped on the surface of the substrate, whichare conveyed forward. The subsequent operations are similar than thoseinvolved in the slurry molding route.

D. Deposition of Selenium on Massive Graphene Surfaces of the PorousGraphitic Structure

Once a layer of porous graphitic structure is prepared, this layer canbe immersed in an electrolyte (preferably liquid electrolyte), whichcomprises a solvent and a selenium source dissolved or dispersed in thesolvent. This layer basically serves as a cathode in an externalelectrochemical deposition chamber or a cathode in an intended Li—Secell (encased inside the packaging or casing of a battery).

Subsequently, an anode layer is also immersed in the chamber, or encasedinside a battery cell. Any conductive material can be used as an anodematerial, but preferably this layer contains some lithium. In such anarrangement, the integral layer of porous graphitic structure and theanode are in ionic contact with the electrolyte. An electric current isthen supplied between the anode and the integral layer of porousgraphitic structure (serving as a cathode) with a sufficient currentdensity for a sufficient period of time to electrochemically depositnanoscaled selenium particles or coating on the graphene surfaces toform the pre-selenized active cathode layer. The required currentdensity depends upon the desired speed of deposition and uniformity ofthe deposited material.

This current density can be readily adjusted to deposit Se particles orcoating that have a thickness or diameter smaller than 20 nm (preferably<10 nm, more preferably <5 nm, and further preferably <3 nm). Theresulting nanoscaled selenium particles or coating occupy a weightfraction of at least 70% (preferably >80%, more preferably >90%, andmost preferably >95%) based on the total weights of the seleniumparticles or coating and the graphene material combined.

In one preferred embodiment, the selenium source is selected fromM_(x)Se_(y), wherein x is an integer from 1 to 3 and y is an integerfrom 1 to 10, and M is a metal element selected from an alkali metal, analkaline metal selected from Mg or Ca, a transition metal, a metal fromgroups 13 to 17 of the periodic table, or a combination thereof. In adesired embodiment, the metal element M is selected from Li, Na, K, Mg,Zn, Cu, Ti, Ni, Co, Fe, or Al. In a particularly desired embodiment,M_(x)Se_(y) is selected from Li₂Se₆, Li₂Se₇, Li₂Se₈, Li₂Se₉, Li₂Se₁₀,Na₂Se₆, Na₂Se₇, Na₂Se₈, Na₂Se₉, Na₂Se₁₀, K₂Se₆, K₂Se₇, K₂Se₈, K₂Se₉, orK₂Se₁₀.

In one embodiment, the anode comprises an anode active material selectedfrom an alkali metal, an alkaline metal, a transition metal, a metalfrom groups 13 to 17 of the periodic table, or a combination thereof.This anode can be the same anode intended for inclusion in a Li—Se cell.

The solvent may be selected from 1,3-dioxolane (DOL),1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME),poly(ethylene glycol) dimethyl ether (PEGDME), diethylene glycol dibutylether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, ethylenecarbonate (EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC),diethyl carbonate (DEC), ethyl propionate, methyl propionate, propylenecarbonate (PC), gamma-butyrolactone (γ-BL), acetonitrile (AN), ethylacetate (EA), propyl formate (PF), methyl formate (MF), toluene, xylene,methyl acetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate(VC), allyl ethyl carbonate (AEC), a hydrofluoroether, a roomtemperature ionic liquid solvent, or a combination thereof.

For the purpose of internal electrochemical deposition of Se on massivegraphene surfaces of a cathode layer in a cell, the electrolyte mayfurther comprise a metal salt selected from lithium perchlorate(LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride(LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluoro-metasulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimidelithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-Fluoroalkyl-Phosphates(LiPF₃(CF₂CF₃)₃), lithium bisperfluoro-ethysulfonylimide (LiBETI),lithium bis(trifluoromethanesulphonyl)imide, lithiumbis(fluorosulphonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI),an ionic liquid-based lithium salt, sodium perchlorate (NaClO₄),potassium perchlorate (KClO₄), sodium hexafluorophosphate (NaPF₆),potassium hexafluorophosphate (KPF₆), sodium borofluoride (NaBF₄),potassium borofluoride (KBF₄), sodium hexafluoroarsenide, potassiumhexafluoroarsenide, sodium trifluoro-metasulfonate (NaCF₃SO₃), potassiumtrifluoro-metasulfonate (KCF₃SO₃), bis-trifluoromethyl sulfonylimidesodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide (NaTFSI), andbis-trifluoromethyl sulfonylimide potassium (KN(CF₃SO₂)₂), or acombination thereof.

In one preferred embodiment, as previously stated above, theelectrochemical deposition is conducted before the cathode active layeris incorporated into a lithium-selenium (Li—Se) battery cell. In otherwords, the anode, the electrolyte, and the integral layer of porousgraphitic structure (serving as a cathode layer) are positioned in anexternal container outside of a lithium-selenium cell. The neededapparatus is similar to an electro-plating system. The step ofelectrochemically depositing nanoscaled selenium particles or coating onthe graphene surfaces is conducted outside the lithium-selenium cell andprior to the battery cell fabrication. After this selenium deposition iscompleted, the pre-selenized integral layer of porous graphiticstructure is then incorporated into the lithium-selenium cell.

In another embodiment, the anode, the electrolyte, and the integrallayer of porous graphitic structure are disposed inside alithium-selenium cell. In other words, the battery cell itself is anelectrochemical deposition system for pre-selenization of the cathodeand the step of electrochemically depositing nanoscaled seleniumparticles or coating on the graphene surfaces occurs after thelithium-selenium cell is fabricated. This electrochemical depositionprocedure is conducted during the first charge cycle of the Li—Se cell.

After an extensive and in-depth research effort, we have come to realizethat such a pre-selenization surprisingly solves several most criticalissues associated with current Li—Se cells. For instance, this methodenables the selenium to be deposited in a thin coating or ultra-fineparticle form, thus, providing ultra-short lithium ion diffusion pathsand, hence, ultra-fast reaction times for fast battery charges anddischarges. This is achieved while maintaining a relatively highproportion of selenium (the active material responsible for storinglithium) and, thus, high specific lithium storage capacity of theresulting cathode active layer in terms of high specific capacity(mAh/g, based on the total weight of the cathode layer, including themasses of the active material, Se, supporting graphene sheets, binderresin, and conductive filler).

It is of significance to note that one might be able to use a prior artprocedure to deposit small Se particles, but not a high Se proportion,or to achieve a high proportion but only in large particles or thickfilm form. But, the prior art procedures have not been able to achieveboth small Se particles and high Se proportion at the same time. This iswhy it is such an unexpected and highly advantageous thing to obtain ahigh selenium loading and yet, concurrently, maintaining anultra-thin/small thickness/diameter of selenium. This has not beenpossible with any prior art selenium loading techniques. For instance,we have been able to deposit nanoscaled selenium particles or coatingthat occupy a >90% weight fraction of the cathode layer and yetmaintaining a coating thickness or particle diameter<3 nm. This is quitea feat in the art of lithium-selenium batteries. As another example, wehave achieved a >95% Se loading at an average Se coating thickness of4.5-7.1 nm.

Electrochemists or materials scientists in the art of Li—Se batterieswould expect that a greater amount of highly conducting graphene sheetsor graphite flakes (hence, a smaller amount of Se) in the cathode activelayer should lead to a better utilization of Se, particularly under highcharge/discharge rate conditions. Contrary to these expectations, wehave observed that the key to achieving a high Se utilization efficiencyis minimizing the Se coating thickness or Se particle size and isindependent of the amount of Se loaded into the cathode provided the Secoating or particle thickness/diameter is small enough (e.g. <10 nm, oreven better if <5 nm). The problem here is that it has not been possibleto maintain a thin Se coating or small particle size if Se is higherthan 50% by weight. Here we have further surprisingly observed that thekey to enabling a high specific capacity at the cathode under high rateconditions is to maintain a high Se loading and still keep the Secoating or particle size as small as possible, and this is accomplishedby using the presently invented pre-selenization (selenium pre-loading)method.

The electrons coming from or going out through the external load orcircuit must go through the conductive additives (in a conventionalselenium cathode) or a conductive framework (e.g. exfoliated graphitemesoporous structure or nanostructure of conductive graphene sheets asherein disclosed) to reach the cathode active material. Since thecathode active material (e.g. selenium or lithium poly selenide) is apoor electronic conductor, the active material particle or coating mustbe as thin as possible to reduce the required electron travel distance.

Furthermore, the cathode in a conventional Li—Se cell typically has lessthan 70% by weight of selenium in a composite cathode composed ofselenium and the conductive additive/support. Even when the seleniumcontent in the prior art composite cathode reaches or exceeds 70% byweight, the specific capacity of the composite cathode is typicallysignificantly lower than what is expected based on theoreticalpredictions. For instance, the theoretical specific capacity of seleniumis 675 mAh/g. A composite cathode composed of 70% selenium (Se) and 30%carbon black (CB), without any binder, should be capable of storing upto 675×70%=472.5 mAh/g. Unfortunately, the observed specific capacity istypically less than 75% or 354 mAh/g (often less than 50% or 237 mAh/gin this example) of what could be achieved. In other words, the activematerial utilization rate is typically less than 75% (or even <50%).This has been a major issue in the art of Li—Se cells and there has beenno solution to this problem. Most surprisingly, the implementation ofmassive graphene surfaces associated with a porous graphitic structureas a conductive supporting material for selenium or lithium polyselenide has made it possible to achieve an active material utilizationrate of typically >>80%, more often greater than 90%, and, in manycases, close to 95%-99%.

Still another unexpected result of the instant method is the observationthat thinner Se coating leads to more stable charge/discharge cyclingwith significantly reduced shuttling effect that has been along-standing impediment to full commercialization of Li—Se batteries.We overcome this problem yet, at the same time, achieving a highspecific capacity. In just about all the prior art Li—Se cells, a higherSe loading leads to a faster capacity decay.

The shuttling effect is related to the tendency for selenium or lithiumpoly selenide that forms at the cathode to get dissolved in the solventand for the dissolved lithium poly selenide species to migrate from thecathode to the anode, where they irreversibly react with lithium to formspecies that prevent selenide from returning back to the cathode duringthe subsequent discharge operation of the Li—Se cell (the detrimentalshuttling effect). It seems that the presence of massive graphenesurfaces have been able to prevent or reduce such a dissolution andmigration issue.

The electrolytic salts to be incorporated into a non-aqueous electrolytemay be selected from a lithium salt such as lithium perchlorate(LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride(LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluoro-metasulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimidelithium [LiN(CF₃SO₂)₂], lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphates(LiPF₃(CF₂CF₃)₃), lithium bisperfluoroethysulfonylimide (LiBETI), anionic liquid salt. Among them, LiPF₆, LiBF₄ and LiN(CF₃SO₂)₂ arepreferred. The content of aforementioned electrolytic salts in thenon-aqueous solvent is preferably 0.5 to 3.0 M (mol/L) at the cathodeside and 3.0 to >10 M at the anode side.

The ionic liquid is composed of ions only. Ionic liquids are low meltingtemperature salts that are in a molten or liquid state when above adesired temperature. For instance, a salt is considered as an ionicliquid if its melting point is below 100° C. If the melting temperatureis equal to or lower than room temperature (25° C.), the salt isreferred to as a room temperature ionic liquid (RTIL). The IL salts arecharacterized by weak interactions, due to the combination of a largecation and a charge-delocalized anion. This results in a low tendency tocrystallize due to flexibility (anion) and asymmetry (cation).

A typical and well-known ionic liquid is formed by the combination of a1-ethyl-3-methylimidazolium (EMI) cation and anN,N-bis(trifluoromethane)sulphonamide (TFSI) anion. This combinationgives a fluid with an ionic conductivity comparable to many organicelectrolyte solutions and a low decomposition propensity and low vaporpressure up to ˜300-400° C. This implies a generally low volatility andnon-flammability and, hence, a much safer electrolyte for batteries.

Ionic liquids are basically composed of organic ions that come in anessentially unlimited number of structural variations owing to thepreparation ease of a large variety of their components. Thus, variouskinds of salts can be used to design the ionic liquid that has thedesired properties for a given application. These include, among others,imidazolium, pyrrolidinium and quaternary ammonium salts as cations andbis(trifluoromethanesulphonyl) imide, bis(fluorosulphonyl)imide, andhexafluorophosphate as anions. Based on their compositions, ionicliquids come in different classes that basically include aprotic, proticand zwitterionic types, each one suitable for a specific application.

Common cations of room temperature ionic liquids (RTILs) include, butnot limited to, tetraalkylammonium, di-, tri-, andtetra-alkylimidazolium, alkylpyridinium, dialkyl-pyrrolidinium,dialkylpiperidinium, tetraalkylphosphonium, and trialkylsulfonium.Common anions of RTILs include, but not limited to, BF₄ ⁻, B(CN)₄ ⁻,CH₃BF₃ ⁻, CH2CHBF₃ ⁻, CF₃BF₃ ⁻, C₂F₅BF₃ ⁻, n-C₃F₇BF₃ ⁻, n-C₄F₉BF₃ ⁻, PF₆⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, N(SO₂CF₃)₂ ⁻, N(COCF₃)(SO₂CF₃)⁻, N(SO₂F)₂ ⁻,N(CN)₂ ⁻, C(CN)₃ ⁻, SCN⁻, SeCN⁻, CuCl₂ ⁻, AlCl₄ ⁻, F(HF)_(2.3) ⁻, etc.Relatively speaking, the combination of imidazolium- or sulfonium-basedcations and complex halide anions such as AlCl₄ ⁻, BF₄ ⁻, CF₃CO₂ ⁻,CF₃SO₃ ⁻, NTf₂ ⁻, N(SO₂F)₂ ⁻, or F(HF)_(2.3) ⁻ results in RTILs withgood working conductivities.

RTILs can possess archetypical properties such as high intrinsic ionicconductivity, high thermal stability, low volatility, low (practicallyzero) vapor pressure, non-flammability, the ability to remain liquid ata wide range of temperatures above and below room temperature, highpolarity, high viscosity, and wide electrochemical windows. Theseproperties, except for the high viscosity, are desirable attributes whenit comes to using an RTIL as an electrolyte ingredient (a salt and/or asolvent) in a Li—Se cell.

In one embodiment, the cathode layer may be pre-loaded with up to 30%(preferably <15% and more preferably <10%) of an active material(selenium or lithium poly selenide) prior to the cathode layerfabrication. In yet another embodiment, the cathode layer can contain aconductive filler, such as carbon black (CB), acetylene black (AB),graphite particles, activated carbon, mesoporous carbon, mesocarbonmicrobead (MCMB), carbon nanotube (CNT), carbon nanofiber (CNF), carbonfiber, or a combination thereof.

The anode active material may contain, as an example, lithium metal foilor a high-capacity Si, Sn, or SnO₂ capable of storing a great amount oflithium. The cathode active material may contain pure selenium (if theanode active material contains lithium), lithium poly selenide, or anyselenium-containing compound, molecule, or polymer. If the cathodeactive material includes lithium-containing species (e.g. lithium polyselenide) when the cell is made, the anode active material can be anymaterial capable of storing a large amount of lithium (e.g. Si, Ge, Sn,SnO₂, etc.).

At the anode side, when lithium metal is used as the sole anode activematerial in a Li—Se cell, there is concern about the formation oflithium dendrites, which could lead to internal shorting and thermalrunaway. Herein, we have used two approaches, separately or incombination, to address this dendrite formation issue: one involving theuse of a high-concentration electrolyte at the anode side and the otherthe use of a nanostructure composed of conductive nanofilaments. For thelatter, multiple conductive nanofilaments are processed to form anintegrated aggregate structure, preferably in the form of a closelypacked web, mat, or paper, characterized in that these filaments areintersected, overlapped, or somehow bonded (e.g., using a bindermaterial) to one another to form a network of electron-conductingpathways. The integrated structure has substantially interconnectedpores to accommodate electrolyte. The nanofilament may be selected from,as examples, a carbon nanofiber (CNF), graphite nanofiber (GNF), carbonnanotube (CNT), metal nanowire (MNW), conductive nanofibers obtained byelectro-spinning, conductive electrospun composite nanofibers,nanoscaled graphene platelet (NGP), or a combination thereof. Thenanofilaments may be bonded by a binder material selected from apolymer, coal tar pitch, petroleum pitch, mesophase pitch, coke, or aderivative thereof.

Nanofibers may be selected from the group consisting of an electricallyconductive electrospun polymer fiber, electrospun polymer nanocompositefiber comprising a conductive filler, nanocarbon fiber obtained fromcarbonization of an electrospun polymer fiber, electrospun pitch fiber,and combinations thereof. For instance, a nanostructured electrode canbe obtained by electro-spinning of polyacrylonitrile (PAN) into polymernanofibers, followed by carbonization of PAN. It may be noted that someof the pores in the structure, as carbonized, are greater than 100 nmand some smaller than 100 nm.

The presently invented cathode active layer may be incorporated in oneof at least four broad classes of rechargeable lithium metal cells (or,similarly, for sodium metal or potassium metal cells):

-   -   (A) Lithium metal-selenium with a conventional anode        configuration: The cell contains an optional cathode current        collector, a presently invented cathode layer, a        separator/electrolyte, and an anode current collector. Potential        dendrite formation may be overcome by using the        high-concentration electrolyte at the anode.    -   (B) Lithium metal-selenium cell with a nanostructured anode        configuration: The cell contains an optional cathode current        collector, a cathode herein invented, a separator/electrolyte,        an optional anode current collector, and a nanostructure to        accommodate lithium metal that is deposited back to the anode        during a charge or re-charge operation. This nanostructure (web,        mat, or paper) of nanofilaments provide a uniform electric field        enabling uniform Li metal deposition, reducing the propensity to        form dendrites. This configuration can provide a dendrite-free        cell for a long and safe cycling behavior.    -   (C) Lithium ion-selenium cell with a conventional anode: For        instance, the cell contains an anode composed of anode active        graphite particles bonded by a binder, such as polyvinylidene        fluoride (PVDF) or styrene-butadiene rubber (SBR). The cell also        contains a cathode current collector, a cathode of the instant        invention, a separator/electrolyte, and an anode current        collector; and    -   (D) Lithium ion-selenium cell with a nanostructured anode: For        instance, the cell contains a web of nanofibers coated with Si        coating or bonded with Si nanoparticles. The cell also contains        an optional cathode current collector, a cathode herein        invented, a separator/electrolyte, and an anode current        collector. This configuration provides an ultra-high capacity,        high energy density, and a safe and long cycle life.

In the lithium-ion selenium cell (e.g. as described in (C) and (D)above), the anode active material can be selected from a wide range ofhigh-capacity materials, including (a) silicon (Si), germanium (Ge), tin(Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al),nickel (Ni), cobalt (Co), manganese (Mn), titanium (Ti), iron (Fe), andcadmium (Cd), and lithiated versions thereof; (b) alloys orintermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, or Cd withother elements, and lithiated versions thereof, wherein said alloys orcompounds are stoichiometric or non-stoichiometric; (c) oxides,carbides, nitrides, sulfides, phosphides, selenides, and tellurides ofSi, Ge, Sn, Pb, Sb, Bi, Zn, Al, Fe, Ni, Co, Ti, Mn, or Cd, and theirmixtures or composites, and lithiated versions thereof; (d) salts andhydroxides of Sn and lithiated versions thereof; (e) carbon or graphitematerials and prelithiated versions thereof; and combinations thereof.Non-lithiated versions may be used if the cathode side contains lithiumpolysulfides or other lithium sources when the cell is made.

A possible lithium metal cell may be comprised of an anode currentcollector, an electrolyte phase (optionally but preferably supported bya porous separator, such as a porous polyethylene-polypropyleneco-polymer film), a cathode of the instant invention, and an optionalcathode collector. This cathode current collector is optional becausethe presently invented layer of porous graphene or graphitic structure,if properly designed, can act as a current collector or as an extensionof a current collector.

For a sodium ion-selenium cell or potassium ion-selenium cell, the anodeactive material layer can contain an anode active material selected fromthe group consisting of: (a) Sodium- or potassium-doped silicon (Si),germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc(Zn), aluminum (Al), titanium (Ti), cobalt (Co), nickel (Ni), manganese(Mn), cadmium (Cd), and mixtures thereof; (b) Sodium- orpotassium-containing alloys or intermetallic compounds of Si, Ge, Sn,Pb, Sb, Bi, Zn, Al, Ti, Co, Ni, Mn, Cd, and their mixtures; (c) sodium-or potassium-containing oxides, carbides, nitrides, sulfides,phosphides, selenides, tellurides, or antimonides of Si, Ge, Sn, Pb, Sb,Bi, Zn, Al, Fe, Ti, Co, Ni, Mn, Cd, and mixtures or composites thereof,(d) Sodium or potassium salts; (e) particles of graphite, hard carbon,soft carbon or carbon particles and pre-sodiated versions thereof(pre-doped or pre-loaded with Na), and combinations thereof.

The following examples are presented primarily for the purpose ofillustrating the best mode practice of the present invention and shouldnot be construed as limiting the scope of the present invention.

Example 1: Preparation of Graphene Oxide (GO) and Reduced GO Nanosheetsfrom Natural Graphite Powder and their Paper/Mats (Layers of PorousGraphitic Structure)

Natural graphite from Huadong Graphite Co. (Qingdao, China) was used asthe starting material. GO was obtained by following the well-knownmodified Hummers method, which involved two oxidation stages. In atypical procedure, the first oxidation was achieved in the followingconditions: 1100 mg of graphite was placed in a 1000 mL boiling flask.Then, 20 g of K₂S₂O₈, 20 g of P₂O₅, and 400 mL of a concentrated aqueoussolution of H₂SO₄ (96%) were added in the flask. The mixture was heatedunder reflux for 6 hours and then let without disturbing for 20 hours atroom temperature. Oxidized graphite was filtered and rinsed withabundant distilled water until neutral pH. A wet cake-like material wasrecovered at the end of this first oxidation.

For the second oxidation process, the previously collected wet cake wasplaced in a boiling flask that contains 69 mL of a concentrated aqueoussolution of H₂SO₄ (96%). The flask was kept in an ice bath as 9 g ofKMnO₄ was slowly added. Care was taken to avoid overheating. Theresulting mixture was stirred at 35° C. for 2 hours (the sample colorturning dark green), followed by the addition of 140 mL of water. After15 min, the reaction was halted by adding 420 mL of water and 15 mL ofan aqueous solution of 30 wt % H₂O₂. The color of the sample at thisstage turned bright yellow. To remove the metallic ions, the mixture wasfiltered and rinsed with a 1:10 HCl aqueous solution. The collectedmaterial was gently centrifuged at 2700 g and rinsed with deionizedwater. The final product was a wet cake that contained 1.4 wt % of GO,as estimated from dry extracts. Subsequently, liquid dispersions of GOplatelets were obtained by lightly sonicating wet-cake materials, whichwere diluted in deionized water.

Surfactant-stabilized RGO (RGO-BS) was obtained by diluting the wet-cakein an aqueous solution of surfactants instead of pure water. Acommercially available mixture of cholate sodium (50 wt %) anddeoxycholate sodium (50 wt %) salts provided by Sigma Aldrich was used.The surfactant weight fraction was 0.5 wt %. This fraction was keptconstant for all samples. Sonication was performed using a BransonSonifier S-250A equipped with a 13 mm step disruptor horn and a 3 mmtapered micro-tip, operating at a 20 kHz frequency. For instance, 10 mLof aqueous solutions containing 0.1 wt % of GO was sonicated for 10 minand subsequently centrifuged at 2700 g for 30 min to remove anynon-dissolved large particles, aggregates, and impurities. Chemicalreduction of as-obtained GO to yield RGO was conducted by following themethod, which involved placing 10 mL of a 0.1 wt % GO aqueous solutionin a boiling flask of 50 mL. Then, 10 μL of a 35 wt % aqueous solutionof N₂H₄ (hydrazine) and 70 mL of a 28 wt % of an aqueous solution ofNH₄OH (ammonia) were added to the mixture, which was stabilized bysurfactants. The solution was heated to 90° C. and refluxed for 1 h. ThepH value measured after the reaction was approximately 9. The color ofthe sample turned dark black during the reduction reaction. Thesesuspensions (GO in water and RGO in surfactant water) were then filteredthrough a vacuum-assisted membrane filtration apparatus to obtain GO andRGO paper or mat.

Example 2: Preparation of Discrete Functionalized GO Sheets fromGraphite Fibers and Porous Films of Chemically Functionalized GO

Chopped graphite fibers with an average diameter of 12 μm and naturalgraphite particles were separately used as a starting material, whichwas immersed in a mixture of concentrated seleniumic acid, nitric acid,and potassium permanganate (as the chemical intercalate and oxidizer) toprepare graphite intercalation compounds (GICs). The starting materialwas first dried in a vacuum oven for 24 h at 80° C. Then, a mixture ofconcentrated seleniumic acid, fuming nitric acid, and potassiumpermanganate (at a weight ratio of 4:1:0.05) was slowly added, underappropriate cooling and stirring, to a three-neck flask containing fibersegments. After 5-16 hours of reaction, the acid-treated graphite fibersor natural graphite particles were filtered and washed thoroughly withdeionized water until the pH level of the solution reached 6. Afterbeing dried at 100° C. overnight, the resulting graphite intercalationcompound (GIC) or graphite oxide fiber was re-dispersed in water and/oralcohol to form a slurry.

In one sample, five grams of the graphite oxide fibers were mixed with2,000 ml alcohol solution consisting of alcohol and distilled water witha ratio of 15:85 to obtain a slurry mass. Then, the mixture slurry wassubjected to ultrasonic irradiation with a power of 200 W for variouslengths of time. After 20 minutes of sonication, GO fibers wereeffectively exfoliated and separated into thin graphene oxide sheetswith oxygen content of approximately 23%-31% by weight. Ammonia waterwas added to one pot of the resulting suspension, which wasultrasonicated for another hour to produce NH₂-functionalized grapheneoxide (f-GO). The GO sheets and functionalized GO sheets were separatelydiluted to a weight fraction of 5% and the suspensions were allowed tostay in the container without any mechanical disturbance for 2 days,forming liquid crystalline phase in the water-alcohol liquid whenalcohol is being vaporized at 80° C.

The resulting suspensions containing GO or f-GO liquid crystals werethen cast onto a glass surface using a doctor's blade to exert shearstresses, inducing GO sheet orientations. The resulting GO or f-GOcoating films, after removal of liquid, have a thickness that can bevaried from approximately 10 to 500 μm. The resulting GO film was thensubjected to heat treatments that involve an initial thermal reductiontemperature of 80-350° C. for 8 hours, followed by heat-treating at asecond temperature of 1,500-2,850° C. for different specimens to obtainvarious porous graphitic films.

Example 3: Preparation of Single-Layer Graphene Sheets and PorousGraphene Mats from Mesocarbon Micro-Beads (MCMBs)

Mesocarbon micro-beads (MCMBs) were supplied from China Steel ChemicalCo., Kaohsiung, Taiwan. This material has a density of about 2.24 g/cm³with a median particle size of about 16 μm. In one example, MCMB (10grams) were intercalated with an acid solution (seleniumic acid, nitricacid, and potassium permanganate at a ratio of 4:1:0.05) for 48-96hours. Upon completion of the reaction, the mixture was poured intodeionized water and filtered. The intercalated MCMBs were repeatedlywashed in a 5% solution of HCl to remove most of the sulfate ions. Thesample was then washed repeatedly with deionized water until the pH ofthe filtrate was no less than 4.5. The slurry was then subjectedultrasonication for 10-100 minutes to fully exfoliate and separate GOsheets. TEM and atomic force microscopic studies indicate that most ofthe GO sheets were single-layer graphene when the oxidation treatmentexceeded 72 hours, and 2- or 3-layer graphene when the oxidation timewas from 48 to 72 hours. The GO sheets contain oxygen proportion ofapproximately 35%-47% by weight for oxidation treatment times of 48-96hours.

The suspension was then diluted to approximately 0.5% by weight in acontainer and was allowed to age therein without mechanical disturbance.The suspension was then slightly heated (to 65° C.) to vaporize thewater under a vacuum-pumping condition. The formation of liquidcrystalline phase became more apparent as water was removed and the GOconcentration was increased. The final concentration in this sample wasset at 4% by weight. The dispersion containing liquid crystals of GOsheets was then cast onto a glass surface using a doctor's blade toexert shear stresses, inducing GO sheet orientations. The resulting GOfilms, after removal of liquid, have a thickness that can be varied fromapproximately 10 to 500 μm. The resulting GO compact was then subjectedto heat treatments that typically involve an initial thermal reductiontemperature of 80-500° C. for 1-5 hours, followed by heat-treating at asecond temperature of 1,500-2,850° C.

Example 4: Preparation of Pristine Graphene Sheets/Platelets (0% Oxygen)and the Effect of Pristine Graphene Sheets

Recognizing the possibility of the high defect population in GO sheetsacting to reduce the conductivity of individual graphene plane, wedecided to study if the use of pristine graphene sheets (non-oxidizedand oxygen-free) can lead to a HOGF having a higher thermalconductivity. Pristine graphene sheets were produced by using the directultrasonication or liquid-phase production process.

In a typical procedure, five grams of graphite flakes, ground toapproximately 20 μm or less in sizes, were dispersed in 1,000 mL ofdeionized water (containing 0.1% by weight of a dispersing agent, Zonyl®FSO from DuPont) to obtain a suspension. An ultrasonic energy level of85 W (Branson S450 Ultrasonicator) was used for exfoliation, separation,and size reduction of graphene sheets for a period of 15 minutes to 2hours. The resulting graphene sheets are pristine graphene that havenever been oxidized and are oxygen-free and relatively defect-free. Thesuspension was then filtered via vacuum-assisted filtration to obtainporous graphene paper structures.

Example 5: Preparation of Graphene Fluoride Nanosheets and PorousGraphitic Structure from these Sheets

Several processes have been used by us to produce GF, but only oneprocess is herein described as an example. In a typical procedure,highly exfoliated graphite (HEG) was prepared from intercalated compoundC₂F-xClF₃. HEG was further fluorinated by vapors of chlorine trifluorideto yield fluorinated highly exfoliated graphite (FHEG). Pre-cooledTeflon reactor was filled with 20-30 mL of liquid pre-cooled ClF₃, thereactor was closed and cooled to liquid nitrogen temperature. Then, nomore than 1 g of HEG was put in a container with holes for ClF₃ gas toaccess and situated inside the reactor. In 7-10 days a gray-beigeproduct with approximate formula C₂F was formed.

Subsequently, a small amount of FHEG (approximately 0.5 mg) was mixedwith 20-30 mL of an organic solvent (methanol, ethanol, 1-propanol,2-propanol, 1-butanol, tert-butanol, isoamyl alcohol) and subjected toan ultrasound treatment (280 W) for 30 min, leading to the formation ofhomogeneous yellowish dispersions. Five minutes of sonication was enoughto obtain a relatively homogenous dispersion of few-layer graphenefluoride, but longer sonication times ensured the production of mostlysingle-layer graphene fluoride sheets. Some of these suspension sampleswere subjected to vacuum oven drying to recover separated graphenefluoride sheets. These graphene fluoride sheets were then added into apolymer-solvent or monomer-solvent solution to form a suspension.Various polymers or monomers (or oligomers) were utilized as theprecursor film materials for subsequent carbonization and graphitizationtreatments.

Upon casting on a glass surface with the solvent removed, the dispersionbecame a brownish film formed on the glass surface. When theseGF-reinforced polymer films were heat-treated, fluorine and othernon-carbon elements were released as gases that generated pores in thefilm. The resulting porous graphitic films had physical densities from0.03 to 1.22 g/cm³. These porous graphitic films were then roll-pressedto obtain solid graphitic films (porous graphitic structures) having adensity from 0.7 to 1.5 g/cm³.

Example 6: Preparation of Nitrogenated Graphene Nanosheets and PorousGraphitic Structures

Graphene oxide (GO), synthesized in Example 1, was finely ground withdifferent proportions of urea and the pelletized mixture heated in amicrowave reactor (900 W) for 30 s. The product was washed several timeswith deionized water and vacuum dried. In this method graphene oxidegets simultaneously reduced and doped with nitrogen. The productsobtained with graphene:urea mass ratios of 1:0.5, 1:1 and 1:2 aredesignated as NGO-1, NGO-2 and NGO-3 respectively and the nitrogencontents of these samples were 14.7, 18.2 and 17.5 wt % respectively asfound by elemental analysis. These nitrogenated graphene sheets remaindispersible in water. Two types of dispersions were then prepared. Oneinvolved adding water-soluble polymer (e.g. polyethylene oxide) into thenitrogenated graphene sheet-water dispersion to produce a water-basedsuspension. The other involved drying the nitrogenated graphenesheet-water dispersion to recover nitrogenated graphene sheets, whichwere then added into precursor polymer-solvent solutions to obtainorganic solvent-based suspensions.

The resulting suspensions were then cast, dried, carbonized andgraphitized to produce porous graphitic structures. The carbonizationtemperatures for comparative samples are 900-1,350° C. Thegraphitization temperatures are from 2,200° C. to 2,950° C.

Example 7: Exfoliated Graphite Worms from Natural Graphite

Natural graphite, nominally sized at 45 μm, provided by Asbury Carbons(405 Old Main St., Asbury, N.J. 08802, USA) was milled to reduce thesize to approximately 14 μm. The chemicals used in the present study,including fuming nitric acid (>90%), seleniumic acid (95-98%), potassiumchlorate (98%), and hydrochloric acid (37%), were purchased fromSigma-Aldrich and used as received.

A reaction flask containing a magnetic stir bar was charged withseleniumic acid (360 mL) and nitric acid (180 mL) and cooled byimmersion in an ice bath. The acid mixture was stirred and allowed tocool for 15 min, and graphite (20 g) was added under vigorous stirringto avoid agglomeration. After the graphite powder was well dispersed,potassium chlorate (110 g) was added slowly over 15 min to avoid suddenincreases in temperature. The reaction flask was loosely capped to allowevolution of gas from the reaction mixture, which was stirred for 48hours at room temperature. On completion of the reaction, the mixturewas poured into 8 L of deionized water and filtered. The slurry wasspray-dried to recover an expandable graphite sample. The dried,expandable graphite was quickly placed in a tube furnace preheated to1,000° C. and allowed to stay inside a quartz tube for approximately 40seconds to obtain exfoliated graphite worms. Some of the graphite formswere then roll-pressed to obtain samples of re-compressed exfoliatedgraphite having a range of physical densities (e.g. 0.3 to 1.2 g/cm³).Some of the graphite worms were subjected to low-intensity sonication toproduce expanded graphite flakes. These expanded graphite flakes werethen made into a paper form using the vacuum-assisted filtrationtechnique.

Example 8: Exfoliated Graphite Worms from Various Synthetic GraphiteParticles or Fibers

Additional exfoliated graphite worms were prepared according to the sameprocedure described in Example 1, but the starting graphite materialswere graphite fiber (Amoco P-100 graphitized carbon fiber), graphiticcarbon nanofiber (Pyrograph-III from Applied Science, Inc., Cedarville,Ohio), spheroidal graphite (HuaDong Graphite, QinDao, China), andmesocarbon micro-beads (MCMBs) (China Steel Chemical Co., Taiwan),respectively. These four types of laminar graphite materials wereintercalated and exfoliated under similar conditions as used for Example1 for different periods of time, from 24 hours to 96 hours.

Some amount of the graphite forms was then roll-pressed to obtainsamples of re-compressed exfoliated graphite having a range of physicaldensities (e.g. 0.3 to 1.2 g/cm³). A second amount of the graphite wormswas subjected to low-intensity sonication to produce expanded graphiteflakes. These expanded graphite flakes were then made into a paper form(the porous graphitic structure) using the vacuum-assisted filtrationtechnique.

Example 9: Exfoliated Graphite Worms from Natural Graphite Using HummersMethod

Additional graphite intercalation compound (GIC) was prepared byintercalation and oxidation of natural graphite flakes (original size of200 mesh, from Huadong Graphite Co., Pingdu, China, milled toapproximately 15 μm) with seleniumic acid, sodium nitrate, and potassiumpermanganate according to the method of Hummers [U.S. Pat. No.2,798,878, Jul. 9, 1957]. In this example, for every 1 gram of graphite,we used a mixture of 22 ml of concentrated seleniumic acid, 2.8 grams ofpotassium permanganate, and 0.5 grams of sodium nitrate. The graphiteflakes were immersed in the mixture solution and the reaction time wasapproximately three hours at 30° C. It is important to caution thatpotassium permanganate should be gradually added to seleniumic acid in awell-controlled manner to avoid overheat and other safety issues. Uponcompletion of the reaction, the mixture was poured into deionized waterand filtered. The sample was then washed repeatedly with deionized wateruntil the pH of the filtrate was approximately 5. The slurry wasspray-dried and stored in a vacuum oven at 60° C. for 24 hours. Theresulting GIC was exposed to a temperature of 1,050° C. for 35 secondsin a quartz tube filled with nitrogen gas to obtain worms of exfoliatedgraphite flakes.

Some of the graphite forms were then roll-pressed to obtain samples ofre-compressed exfoliated graphite having a range of physical densities(e.g. 0.3 to 1.2 g/cm³). Some of the graphite worms were subjected tolow-intensity sonication to produce expanded graphite flakes. Theseexpanded graphite flakes were then made into a paper form using thevacuum-assisted filtration technique.

Example 10: Conductive Web of Filaments from Electrospun PAA Fibrils asa Supporting Layer for the Anode

Poly(amic acid) (PAA) precursors for spinning were prepared bycopolymerizing of pyromellitic dianhydride (Aldrich) and4,4′-oxydianiline (Aldrich) in a mixed solvent oftetrahydrofurane/methanol (THF/MeOH, 8/2 by weight). The PAA solutionwas spun into fiber web using an electrostatic spinning apparatus. Theapparatus consisted of a 15 kV DC power supply equipped with thepositively charged capillary from which the polymer solution wasextruded, and a negatively charged drum for collecting the fibers.Solvent removal and imidization from PAA were performed concurrently bystepwise heat treatments under air flow at 40° C. for 12 h, 100° C. for1 h, 250° C. for 2 h, and 350° C. for 1 h. The thermally cured polyimide(PI) web samples were carbonized at 1,000° C. to obtain carbonizednanofibers with an average fibril diameter of 67 nm. Such a web can beused as a conductive substrate for an anode active material. We observethat the implementation of a network of conductive nanofilaments at theanode of a Li—Se cell can effectively suppress the initiation and growthof lithium dendrites that otherwise could lead to internal shorting.

Example 11: Electrochemical Deposition of Se on Various Webs or PaperStructures (External Electrochemical Deposition) for Li—Se, Na—Se, andK—Se Batteries

The electrochemical deposition may be conducted before the cathodeactive layer is incorporated into an alkali metal-selenium battery cell(Li—Se, Na—Se, or K—Se cell). In this approach, the anode, theelectrolyte, and the integral layer of porous graphitic structure(serving as a cathode layer) are positioned in an external containeroutside of a lithium-selenium cell. The needed apparatus is similar toan electro-plating system, which is well-known in the art.

In a typical procedure, a metal polyselenide (M_(x)Se_(y)) is dissolvedin a solvent (e.g. mixture of DOL/DME in a volume ratio from 1:3 to 3:1)to form an electrolyte solution. An amount of a lithium salt may beoptionally added, but this is not required for external electrochemicaldeposition. A wide variety of solvents can be utilized for this purposeand there is no theoretical limit to what type of solvents can be used;any solvent can be used provided that there is some solubility of themetal polyselenide in this desired solvent. A greater solubility wouldmean a larger amount of selenium can be derived from the electrolytesolution.

The electrolyte solution is then poured into a chamber or reactor undera dry and controlled atmosphere condition (e.g. He or Nitrogen gas). Ametal foil can be used as the anode and a layer of the porous graphiticstructure as the cathode; both being immersed in the electrolytesolution. This configuration constitutes an electrochemical depositionsystem. The step of electrochemically depositing nanoscaled seleniumparticles or coating on the graphene surfaces is conducted at a currentdensity preferably in the range from 1 mA/g to 10 A/g, based on thelayer weight of the porous graphitic structure.

The chemical reactions that occur in this reactor may be represented bythe following equation: M_(x)Se_(y)→M_(x)Se_(y-z)+zSe (typically z=1-4).Quite surprisingly, the precipitated Se is preferentially nucleated andgrown on massive graphene surfaces to form nanoscaled coating ornanoparticles. The coating thickness or particle diameter and the amountof Se coating/particles may be controlled by the specific surface area,electro-chemical reaction current density, temperature and time. Ingeneral, a lower current density and lower reaction temperature lead toa more uniform distribution of Se and the reactions are easier tocontrol. A longer reaction time leads to a larger amount of Se depositedon graphene surfaces and the reaction is ceased when the selenium sourceis consumed or when a desired amount of Se is deposited.

Example 12: Electrochemical Deposition of Se on Various Webs orPaper-Based Cathode Structures in Li—Se, Na—Se, or K—Se Batteries(Internal Electrochemical Deposition)

As an alternative to the external electrochemical deposition, aninternal electrochemical conversion and deposition of Se from anelectrolyte-borne selenium source onto massive graphene surfaces wasalso conducted using a broad array of porous graphitic structures. As atypical procedure, the anode, the electrolyte, and the integral layer ofporous graphitic structure are packaged inside a housing to form alithium-selenium cell. In such a configuration, the battery cell itselfis an electrochemical deposition system for pre-selenization of thecathode and the step of electrochemically depositing nanoscaled seleniumparticles or coating on the graphene surfaces occurs after thelithium-selenium cell is fabricated and conducted during the firstcharge cycle of the Li—Se cell.

As a series of examples, lithium polyselenide (Li_(x)Se_(y))— and sodiumpolyselenide (Na_(x)Se_(y))-containing electrolytes with desired x and yvalues (e.g. x=2, and y=6-10) dissolved in solvent were prepared bychemically reacting stoichiometric amounts of selenium and Li₂Se orNa₂Se in polyselenide free electrolyte of 0.5 M LiTFSI+0.2 M LiNO₃ (or0.5 M NaTFSI+0.2 M NaNO₃) in DOL/DME (1:1, v:v). The electrolyte wasstirred at 75° C. for 3-7 hours and then at room temperature for 48hours. The resulting electrolytes contain different Li_(x)Se_(y) orNa_(x)Se_(y) species (e.g. x=2, and y=6-10, depending upon reactiontimes and temperatures), which are intended for use as a selenium sourcein a battery cell.

In a Li—Se or Na—Se cell, one of these electrolytes was selected tocombine with an anode current collector (Cu foil), an anode layer (e.g.Li metal foil or Na particles), a porous separator, a layer of porousgraphitic structure, and a cathode current collector (Al foil) to form aLi—Se or room temperature Na—Se cell. The cell was then subjected to afirst charge procedure using a current density ranging from 5 mA/g to 50A/g. The best current density range was found to be from 50 mA/g to 5A/g.

Examples of the metal polysulfide (M_(x)Se_(y)) materials, solvents,graphene materials, and exfoliated graphite materials used in thepresent study are presented in Table 1 below:

TABLE 1 Selected examples of the metal polysulfide materials, solvents,graphene materials, and exfoliated graphite materials used in thepresent study. Selenium source (e.g. Type of porous graphitic structureM_(x)Se_(y)) Solvent Li/Na/K salts in the cathode Li₂Se₆ DOL/DME LiTFSIPristine graphene, GO, GRO, graphene fluoride Li₂Se₉ DOL/DME LiTFSINitrogenated graphene, graphite worms, expanded graphite Na₂Se₅ Tetraethylene NaTFSI Pristine graphene, GO, GRO, glycol dimethyl graphenefluoride, NH₂- ether (TEGDME) functionalized graphene Na₂Se₆ TEGDMENaTFSI RGO, graphite worms, expanded graphite K₂Se₆ TEGDME KTFSI RGO,graphite worms, expanded graphite MgSe₆ Diglyme/tetraglyme[Mg₂Cl₃][HMDSAlCl₃] Pristine graphene, GRO, graphite (HMDS = worms,expanded graphite hexamethyldisilazide) MgSe₄ Diglyme/tetraglyme[Mg₂Cl₃][HMDSAlCl₃] Pristine graphene, GRO, graphite (HMDS = worms,expanded graphite hexamethyldisilazide) CuSe₂ NH₄OH or HCl or CuCl₂Pristine graphene, GRO, graphite H₂SO₄ worms, expanded graphite Cu₈Se₅NH₄OH or HCl or CuCl₂ Pristine graphene, GRO, graphite H₂SO₄ worms,expanded graphite ZnSe H₂SO₄ solution ZnSO₄ Pristine graphene, GRO,graphite worms, expanded graphite Al₂Se₃ H₂SO₄ Al₂(SO₄)₃ RGO, graphiteworms SnSe₂ HNO₃ and HCl SnCl₂ Pristine graphene, expanded graphite SnSeHCl SnCl₂ Pristine graphene, GRO, graphite worms, expanded graphite

Several series of Li metal, Na metal, Na-ion, and Li-ion cells wereprepared using the presently prepared cathode. The first series is a Lior Na metal cell containing a copper foil as an anode current collectorand the second series is also a Li or Na metal cell having ananostructured anode of conductive filaments (based on electrospuncarbon fibers) plus a copper foil current collector. The third series isa Li-ion cell having a nanostructured anode of conductive filaments(based on electrospun carbon fibers coated with a thin layer of Si usingCVD) plus a copper foil current collector. The fourth series is a Li-ioncell having a graphite-based anode active material as an example of themore conventional anode.

Comparative Examples 12a: Mixing of Selenium with Graphene Sheets orActivated Carbon Particles Via Ball-Milling

Selenium particles and graphene sheets (0% to 49% by weight of Se in theresulting composite) were physically blended and then subjected to ballmilling for 2-24 hours to obtain Se-graphene composite particles(typically in a ball or potato shape). For comparison, graphene sheetsonly (without Se) were also ball-milled to obtain ball- or potato-shapedgraphene particles. The particles, containing various Se contents, werethen made into a layer of graphene structure intended for use in thecathode. Another series of samples for comparison were made undersimilar processing conditions, but with activated carbon particlesreplacing graphene sheets.

Examples 13: Some Examples of Electrolytes Used

A wide range of lithium salts can be dissolved in a wide array ofsolvents, individually or in a mixture form. Both ether- andcarbonate-based solvents are suitable for use in an electrolyte for aLi—Se cell. The following are good choices for lithium salts that aredissolved well to a high concentration in selected solvents: lithiumborofluoride (LiBF₄), lithium trifluoro-metasulfonate (LiCF₃SO₃),lithium bis-trifluoromethyl sulfonylimide (LiN(CF₃SO₂)₂ or LITFSI),lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate(LiBF₂C₂O₄), and lithium bisperfluoroethy-sulfonylimide (LiBETI). Theseselected solvents are DME/DOL mixture, TEGDME/DOL, PEGDME/DOL, andTEGDME. A good electrolyte additive for helping to stabilize Li metal isLiNO₃. Useful sodium salts and potassium salts include sodiumperchlorate (NaClO₄), potassium perchlorate (KClO₄), sodiumhexafluorophosphate (NaPF₆), potassium hexafluorophosphate (KPF₆),sodium borofluoride (NaBF₄), potassium borofluoride (KBF₄), sodiumhexafluoroarsenide, potassium hexafluoroarsenide, sodiumtrifluoro-metasulfonate (NaCF₃SO₃), potassium trifluoro-metasulfonate(KCF₃SO₃), bis-trifluoromethyl sulfonylimide sodium (NaN(CF₃SO₂)₂),sodium trifluoromethanesulfonimide (NaTFSI), and bis-trifluoromethylsulfonylimide potassium (KN(CF₃SO₂)₂). Good solvents are DME/DOLmixture, TEGDME/DOL, PEGDME/DOL, and TEGDME.

Room temperature ionic liquids (RTILs) are of great interest due totheir low volatility and non-flammability. Particularly useful ionicliquid-based electrolyte systems include: lithium bis(trifluoromethanesulfonyl)imide in a N-n-butyl-N-ethylpyrrolidiniumbis(trifluoromethane sulfonyl)imide (LiTFSI in BEPyTFSI),N-methyl-N-propylpiperidinium bis(trifluoromethyl sulfonyl)imide(PP₁₃TFSI) containing LiTFSI, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide (DEMETFSI) containing LiTFSI.

Examples 14: Evaluation of Electrochemical Performance of Various Li—Se,Na—Se, and K—Se Cells

Charge storage capacities were measured periodically and recorded as afunction of the number of cycles. The specific discharge capacity hereinreferred to is the total charge inserted into the cathode during thedischarge, per unit mass of the composite cathode (counting the weightsof cathode active material, conductive additive or support, binder, andany optional additive combined). The specific charge capacity refers tothe amount of charges per unit mass of the composite cathode. Thespecific energy and specific power values presented in this section arebased on the total cell weight. The morphological or micro-structuralchanges of selected samples after a desired number of repeated chargingand recharging cycles were observed using both transmission electronmicroscopy (TEM) and scanning electron microscopy (SEM).

Shown in FIG. 3 are charge and discharge cycling results of three Li—Secells, one containing a presently invented cathode structure prepared bythe external electrochemical deposition of selenium into a layer ofporous recompressed exfoliated graphite paper, the second cellcontaining a cathode prepared by using PVD to deposit selenium in acomparable sheet of recompressed exfoliated graphite paper, and thethird containing a cathode material prepared by ball-milling a mixtureof Se powder and activated carbon powder. The discharge capacities ofthe above two cells are plotted as a function of the number ofcharge-discharge cycles. The cathode layers in the two Li—Se cells weredesigned to have approximately 65% by weight of Se deposited therein.Presumably, the resulting composite or hybrid cathode of each cellshould exhibit a maximum specific capacity of 675×65%=439 mAh/g.However, the cathode layer prepared by PVD deposition of Se exhibits aspecific capacity of only 345 mAh/g, which means a Se utilizationefficiency of 345/439=78.6%. The cathode layer prepared by ball millingof AC-Se powder mixture exhibits a specific capacity of only 301 mAh/g,which means a Se utilization efficiency of 301/439=68%. In contrast, thecathode layer having electrochemically deposited Se coating (thickness8.2 nm) prepared according to an embodiment of the instant inventiondelivers an Se utilization efficiency of 388/439=88%. This dramaticdifference in efficiency is truly stunning. Data from many more samplesinvestigated are summarized in Table 2 below:

TABLE 2 Selenium utilization efficiency data for alkali metal-seleniumcell cathodes containing various Se contents, Se coating thicknesses orparticle diameters, substrate materials, and Se deposition methods.Cathode Discharge % of Se and discharge capacity, Se Sample Cathodeactive thickness or capacity mAh/g, based utilization ID layer materialdiameter (nm) (mAh/g) on Se weight efficiency PG-1 Pristine 95% Se; 6.1nm 585 615.8 91.23% graphene PG-2 Pristine 95% Se; 3.5 nm 604 635.894.19% graphene PG-3 Pristine 75% Se; 7.5 nm 454 605.3 89.68% graphenePG-C-1 Pristine 75% Se (PVD) + 296 394.7 58.47% graphene PG PG-C-2Pristine 75% Se + PG; 205 273.3 40.49% graphene ball-milled PG-C-3Carbon black 75% Se + CB; 170 226.7 33.58% ball-milled GO-1 GO 70% Se,432 617.1 91.43% External GO-1C GO 70% Se, Chem 364 520.0 77.04%reaction RGO-1 RGO 70% Se, 434 620.0 91.85% External RGO-C RGO 70% Se,Chem 360 514.3 76.19% reaction NGO-1 NGO 70% Se, 422 602.9 89.31%External NGO-2 NGO 70% Se, in a 416 594.3 88.04% cell f-GO-1 f-GO 65%Se; 9.0 nm, 401 616.9 91.40% in a cell f-GO-2 f-GO 80% Se; 9.1 nm, 476595.0 88.15% in a cell f-GO-3 f-GO 95% Se; 4.8 nm, 589 620.0 91.85% in acell EG-1 Exfoliated 65% Se; 18.2 397 610.8 90.48% graphite worms nm,external EG-2 Exfoliated 80% Se; 16.8 478 597.5 88.52% graphite wormsnm, external EG-3 Exfoliated 90% Se; 17 nm, 521 578.9 85.76% graphiteworms external EG-1C Exfoliated 65% Se; ball- 216 332.3 49.23% graphiteworms milled EP-1 Expanded 65% Se; 16.4 388 596.9 88.43% graphite flakesnm, in a cell EP-2 Expanded 80% Se; 17.7 459 573.8 85.00% graphiteflakes nm, in a cell EP-3 Expanded 80% Se; 20.5 448 560.0 82.96%graphite flakes nm, external

The following observations can be made from the data of Table 2:

-   -   1) Thinner coatings prepared according to the instant invention        lead to higher efficiency of Se utilization given comparable Se        proportion. Given comparable Se coating thickness, the Se        utilization efficiency is relatively independent of the S        proportion deposited on graphene surfaces of a graphene material        or exfoliated graphite material.    -   2) The presently invented electrochemical deposition method is        significantly more effective than all conventional methods (PVD        deposition, ball-milling, chemical reaction-based deposition,        etc.) in terms of imparting Se utilization efficiency to the        resulting cathode structure of a Li—Se, Na—Se, or K—Se cell.    -   3) Although both exfoliated graphite materials and graphene        materials are very effective in imparting Se utilization        efficiency, graphene materials (pristine graphene, GO, RGO, NGO,        fGO, etc.) are relatively more effective than exfoliated        graphite materials (graphite worms, expanded graphite flakes,        etc.). Among various graphene materials, the efficiency is        ranked as follows: Pristine graphene>fGO>NGO>RGO>GO. This is        unexpected considering the notion that GO possess functional        groups capable of binding with selenium.    -   4) Both external electrochemical deposition and internal        electrochemical deposition are capable of depositing a high Se        proportion while maintaining a thin Se coating (hence, high Se        utilization efficiency). Prior art methods are not capable of        achieving both.

The data in FIG. 3 further indicate that the presently invented Li—Secell does not exhibit any significant decay (only 8.5%) even after 550cycles. In contrast, the prior art cell containing PVD deposited Scoating-based cathode suffers a 31.3% capacity decay after 550 cycles.The cell featuring a cathode containing ball-milled Se/AC powder suffersa 43.5% capacity decay after 550 cycles. In fact, it suffers a 20%capacity decay after 240 cycles. The cycle life of a lithium batterycell is usually defined as the number of cycles when the cell reaches a20% capacity decay. With this definition, the prior art Li—Se cellfeaturing a ball-milled Se/AC cathode shows a cycle life of 240 cycles.These results are quite unexpected considering that the same type ofgraphene was used as the supporting material and the same amount ofselenium was deposited in these three cell cathodes.

FIG. 4 shows Ragone plots (cell power density vs. cell energy density)of five Li metal-selenium cells. The presently invented Li—Se cellfeaturing a pristine graphene (PG)-based cathode containingelectrochemically deposited selenium particles (87% by weight of S)exhibits an exceptional cell energy density (as high as 801 Wh/kg, basedon total cell weight), which has not been previously achieved by anyprior art Li—Se cell. The same cell also delivers a maximum powerdensity as high as 2915 W/kg, which is significantly higher than typicalpower densities (up to 500 W/kg) of lithium-ion batteries. Anotherinventive Li—Se cell, featuring a PG-based cathode containingelectrochemically deposited selenium particles (64% Se), exhibits anoutstanding maximum cell energy density of 662 Wh/kg and maximum powerdensity of 2,652 W/kg. These are also an unprecedented combination ofhigh energy density and high power density.

In contrast, the PG-based cathode containing chemically depositedselenium particles (64% Se) enables the Li—Se cell to store up to 397Wh/kg and delivers a maximum power density of 1,558 W/kg. These aresignificantly lower than those of the presently invented cells. The cellfeaturing a PG-based cathode containing solution-deposited seleniumparticles (64% Se) exhibits a maximum energy density of 357 Wh/kg andmaximum power density of 1,488 W/kg. The CNT-based cathode containingsolution-deposited selenium particles (64% Se) delivers a maximum cellenergy density of 285 Wh/kg and maximum power density of 1,628 W/kg.These data have clearly demonstrated the unexpected yet superioreffectiveness of the presently invented external and internalelectrochemical deposition methods.

Shown in FIG. 6 are Ragone plots (cell power density vs. cell energydensity) for 4 alkali metal-selenium cells. The first cell is a Na—Secell featuring a RGO-based cathode containing electrochemicallydeposited selenium particles (70% Se), which exhibits the highest energydensity and power density among the four cells. The second is a Na—Secell featuring a RGO-based cathode containing chemically depositedselenium particles (70% Se). Clearly, the cathode having chemicallydeposited Se is not as effective as the presently invented cathode ofelectrochemically deposited Se in providing high energy density andpower density. The third cell is a K—Se cell featuring a RGO-basedcathode containing electrochemically deposited selenium particles (70%Se), and the fourth cell is a K—Se cell featuring a RGO-based cathodecontaining solution-deposited selenium particles (70% Se). Again, thepresently invented electrochemical method is so much superior. The datain FIG. 6 also indicate that the presently invented Na—Se cells canstore an energy density up to 287 Wh/kg, which is significantly higherthan those of Li-ion batteries. Additionally, even K—Se cells can storeup to 180 Wh/kg, better than most of the Li-ion cells. These highlysurprising results are a good testament to the effectiveness of thepresently invented method of depositing selenium on graphene surfaces.

In summary, the present invention provides an innovative, versatile, andsurprisingly effective platform materials technology that enables thedesign and manufacture of superior alkali metal-selenium rechargeablebatteries. The alkali metal-selenium cell featuring a cathode containinga graphene or exfoliated graphite structure with ultra-thin seleniumelectrochemically deposited thereon exhibits a high cathode activematerial utilization rate, high specific capacity, high specific energy,high power density, little or no shuttling effect, and long cycle life.When a similarly configured anode structure (with no selenium) or ananostructured carbon filament web is implemented at the anode tosupport a lithium film (e.g. foil), the lithium dendrite issue is alsosuppressed or eliminated.

1. An electrochemical method of producing a pre-selenized active cathodelayer for a rechargeable alkali metal-selenium cell, said methodcomprising: (a) preparing an integral layer of porous graphiticstructure having massive graphene surfaces with a specific surface areagreater than 100 m²/g, wherein said porous graphitic structure comprisesa graphene material selected from pristine graphene, graphene oxide,reduced graphene oxide, graphene fluoride, graphene chloride, graphenebromide, graphene iodide, hydrogenated graphene, nitrogenated graphene,boron-doped graphene, nitrogen-doped graphene, chemically functionalizedgraphene, or a combination thereof; an exfoliated graphite materialselected from exfoliated graphite worms, expanded graphite flakes, orrecompressed graphite worms or flakes; or a mixture of graphene materialand exfoliated graphite material, and wherein said porous graphiticstructure comprises multiple sheets of said graphene material ormultiple flakes of said exfoliated graphite material that areintersected or interconnected to form said integral layer and mayfurther comprise an optional binder of 0-10% by weight, an optionalconductive filler included in said integral layer, or an optionalselenium or selenium-containing compound pre-loaded therein at 0 to 49%by weight; (b) preparing an electrolyte comprising a non-aqueous solventand a selenium source dissolved or dispersed in said solvent; (c)preparing an anode; and (d) bringing said integral layer of porousgraphitic structure and said anode in ionic contact with saidelectrolyte and imposing an electric current between said anode and saidintegral layer of porous graphitic structure, serving as a cathode, witha sufficient current density for a sufficient period of time toelectrochemically deposit nanoscaled selenium particles or coatingdirectly on said graphene surfaces to form said pre-selenized activecathode layer, wherein said particles or coating have a thickness ordiameter smaller than 20 nm.
 2. The method of claim 1, wherein saidselenium source is selected from M_(x)Se_(y), wherein x is an integerfrom 1 to 3 and y is an integer from 1 to 10, and M is a metal elementselected from an alkali metal, an alkaline metal selected from Mg or Ca,a transition metal, a metal from groups 13 to 17 of the periodic table,or a combination thereof.
 3. The method of claim 1, wherein said anodecomprises an anode active material selected from an alkali metal, analkaline metal, a transition metal, a metal from groups 13 to 17 of theperiodic table, or a combination thereof.
 4. The method of claim 2,wherein said metal element M is selected from Li, Na, K, Mg, Zn, Cu, Ti,Ni, Co, Fe, or Al.
 5. The method of claim 2, wherein said M_(x)Se_(y) isselected from Li₂Se₆, Li₂Se₇, Li₂Se₈, Li₂Se₉, Li₂Se₁₀, Na₂Se₆, Na₂Se₇,Na₂Se₈, Na₂Se₉, Na₂Se₁₀, K₂Se₆, K₂Se₇, K₂Se₈, K₂Se₉, K₂Se₁₀, or acombination thereof.
 6. The method of claim 1, further comprising aprocedure of depositing an element Z to said porous graphitic structurewherein said element Z is mixed with selenium or formed as discrete Zcoating or particles having a dimension less than 100 nm and said Zelement is selected from Sn, Sb, Bi, S, Te, or a combination thereof andthe weight of element Z is less than the weight of selenium.
 7. Themethod of claim 6, wherein said procedure of depositing element Zincludes electrochemical deposition, chemical deposition, or solutiondeposition.
 8. The method of claim 1, wherein said nanoscaled seleniumparticles or coating occupy a weight fraction of at least 70% based onthe total weights of said selenium particles or coating and saidgraphene material combined.
 9. The method of claim 1, wherein saidelectrolyte further comprises a metal salt selected from lithiumperchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithiumborofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluoro-metasulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimidelithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphates(LiPF₃(CF₂CF₃)₃), lithium bisperfluoro-ethysulfonylimide (LiBETI),lithium bis(trifluoromethanesulphonyl)imide, lithiumbis(fluorosulphonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI),an ionic liquid-based lithium salt, sodium perchlorate (NaClO₄),potassium perchlorate (KClO₄), sodium hexafluorophosphate (NaPF₆),potassium hexafluorophosphate (KPF₆), sodium borofluoride (NaBF₄),potassium borofluoride (KBF₄), sodium hexafluoroarsenide, potassiumhexafluoroarsenide, sodium trifluoro-metasulfonate (NaCF₃SO₃), potassiumtrifluoro-metasulfonate (KCF₃SO₃), bis-trifluoromethyl sulfonylimidesodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide (NaTFSI),bis-trifluoromethyl sulfonylimide potassium (KN(CF₃SO₂)₂), or acombination thereof.
 10. The method of claim 1, wherein said solvent isselected from 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME),tetraethylene glycol dimethylether (TEGDME), poly(ethylene glycol)dimethyl ether (PEGDME), diethylene glycol dibutyl ether (DEGDBE),2-ethoxyethyl ether (EEE), sulfone, sulfolane, ethylene carbonate (EC),dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate(DEC), ethyl propionate, methyl propionate, propylene carbonate (PC),gamma-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA),propyl formate (PF), methyl formate (MF), toluene, xylene, methylacetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate (VC),allyl ethyl carbonate (AEC), a hydrofluoroether, a room temperatureionic liquid solvent, or a combination thereof.
 11. The method of claim1, wherein said anode, said electrolyte, and said integral layer ofporous graphitic structure are disposed in an external container outsideof a lithium-selenium cell and said step of electrochemically depositingnanoscaled selenium particles or coating on said graphene surfaces isconducted outside said lithium-selenium cell and said method furtherincludes a step of incorporating said pre-selenized active cathode layerin said lithium-selenium cell.
 12. The method of claim 1, wherein saidanode, said electrolyte, and said integral layer of porous graphiticstructure are disposed inside a lithium-selenium cell and said step ofelectrochemically depositing nanoscaled selenium particles or coating onsaid graphene surfaces is conducted after said lithium-selenium cell isproduced.
 13. The method of claim 1, wherein said anode, saidelectrolyte, and said integral layer of porous graphitic structure arepart of a lithium-selenium cell and said step of electrochemicallydepositing nanoscaled selenium particles or coating on said graphenesurfaces occurs after said lithium-selenium cell is fabricated and isconducted during a first charge cycle of said cell.
 14. The method ofclaim 1, wherein said nanoscaled selenium particles or coating occupy aweight fraction of at least 80%.
 15. The method of claim 1, wherein saidnanoscaled selenium particles or coating occupy a weight fraction of atleast 90%.
 16. The method of claim 1, wherein said nanoscaled seleniumparticles or coating have a thickness or diameter smaller than 10 nm.17. The method of claim 1, wherein said nanoscaled selenium particles orcoating have a thickness or diameter smaller than 5 nm.
 18. The methodof claim 1, wherein said nanoscaled selenium particles or coating have athickness or diameter smaller than 3 nm.
 19. A pre-selenized activecathode layer produced by the method of claim 1 for a rechargeablealkali metal-selenium cell selected from lithium-selenium cell,sodium-selenium cell, or potassium-selenium cell, wherein said graphenesheets are chemically bonded together with an adhesive resin.
 20. Arechargeable alkali metal-selenium cell comprising an anode activematerial layer, an optional anode current collector, a porous separatorand/or an electrolyte, the pre-selenized active cathode layer of claim19, and an optional cathode current collector, wherein said alkalimetal-selenium cell is selected from lithium-selenium cell,sodium-selenium cell, or potassium-selenium cell.
 21. The rechargeablealkali metal-selenium cell of claim 20 wherein said electrolyte isselected from polymer electrolyte, polymer gel electrolyte, compositeelectrolyte, ionic liquid electrolyte, non-aqueous liquid electrolyte,soft matter phase electrolyte, solid-state electrolyte, or a combinationthereof.
 22. The rechargeable alkali metal-selenium cell of claim 20wherein said electrolyte contains an alkali salt selected from lithiumperchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithiumborofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluoro-metasulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimidelithium (LiN(CF₃SO₂)₂, lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphates(LiPF3(CF₂CF₃)₃), lithium bisperfluoroethysulfonylimide (LiBETI), anionic liquid salt, sodium perchlorate (NaClO₄), potassium perchlorate(KClO₄), sodium hexafluorophosphate (NaPF₆), potassiumhexafluorophosphate (KPF₆), sodium borofluoride (NaBF₄), potassiumborofluoride (KBF₄), sodium hexafluoroarsenide, potassiumhexafluoroarsenide, sodium trifluoro-metasulfonate (NaCF₃SO₃), potassiumtrifluoro-metasulfonate (KCF₃SO₃), bis-trifluoromethyl sulfonylimidesodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide (NaTFSI),bis-trifluoromethyl sulfonylimide potassium (KN(CF₃SO₂)₂), or acombination thereof.
 23. The rechargeable alkali metal-selenium cell ofclaim 20 wherein said solvent is selected from ethylene carbonate (EC),dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate(DEC), ethyl propionate, methyl propionate, propylene carbonate (PC),gamma.-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA),propyl formate (PF), methyl formate (MF), toluene, xylene or methylacetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate (VC),allyl ethyl carbonate (AEC), 1,3-dioxolane (DOL), 1,2-dimethoxyethane(DME), tetraethylene glycol dimethylether (TEGDME), poly(ethyleneglycol) dimethyl ether (PEGDME), diethylene glycol dibutyl ether(DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, roomtemperature ionic liquid, or a combination thereof.
 24. The rechargeablealkali metal-selenium cell of claim 20, further comprising a layer ofprotective material disposed between said anode and said porousseparator, wherein said protective material is a lithium ion conductor.25. The rechargeable alkali metal-selenium cell of claim 24, whereinsaid protective material consists of a solid electrolyte.
 26. Therechargeable alkali metal-selenium cell of claim 20 wherein said anodeactive material layer contains an anode active material selected fromlithium metal, sodium metal, potassium metal, a lithium metal alloy,sodium metal alloy, potassium metal alloy, a lithium intercalationcompound, a sodium intercalation compound, a potassium intercalationcompound, a lithiated compound, a sodiated compound, a potassium-dopedcompound, lithiated titanium dioxide, lithium titanate, lithiummanganate, a lithium transition metal oxide, Li₄Ti₅O₁₂, or a combinationthereof.
 27. The rechargeable alkali metal-selenium cell of claim 20wherein said cell is a lithium ion-selenium cell and said anode activematerial layer contains an anode active material selected from the groupconsisting of: (a) silicon (Si), germanium (Ge), tin (Sn), lead (Pb),antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), nickel (Ni),cobalt (Co), manganese (Mn), titanium (Ti), iron (Fe), and cadmium (Cd),and lithiated versions thereof; (b) alloys or intermetallic compounds ofSi, Ge, Sn, Pb, Sb, Bi, Zn, Al, or Cd with other elements, and lithiatedversions thereof, wherein said alloys or compounds are stoichiometric ornon-stoichiometric; (c) oxides, carbides, nitrides, sulfides,phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al,Fe, Ni, Co, Ti, Mn, or Cd, and their mixtures or composites, andlithiated versions thereof; (d) salts and hydroxides of Sn and lithiatedversions thereof; (e) carbon or graphite materials and prelithiatedversions thereof; and combinations thereof.
 28. The rechargeable alkalimetal-selenium cell of claim 20, wherein said cell is a sodiumion-selenium cell or potassium ion-selenium cell and said anode activematerial layer contains an anode active material selected from the groupconsisting of: (a) sodium- or potassium-doped silicon (Si), germanium(Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn),aluminum (Al), titanium (Ti), cobalt (Co), nickel (Ni), manganese (Mn),cadmium (Cd), and mixtures thereof; (b) sodium- or potassium-containingalloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti,Co, Ni, Mn, Cd, and their mixtures; (c) sodium- or potassium-containingoxides, carbides, nitrides, sulfides, phosphides, selenides, tellurides,or antimonides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Fe, Ti, Co, Ni, Mn,Cd, and mixtures or composites thereof, (d) sodium or potassium salts;(e) particles of graphite, hard carbon, soft carbon or carbon particlesand pre-sodiated versions thereof; and combinations thereof.
 29. Therechargeable alkali metal-selenium cell of claim 20 wherein saidgraphene material comprises nanographene sheets or platelets with athickness less than 10 nm.
 30. The rechargeable alkali metal-seleniumcell of claim 20 wherein said graphene material comprises nanographenesheets or platelets selected from single-layer or few-layer pristinegraphene, wherein few-layer is defined as 10 planes of hexagonal carbonatoms or less.
 31. The rechargeable alkali metal-selenium cell of claim20 wherein said binder material is selected from a resin, a conductivepolymer, coal tar pitch, petroleum pitch, mesophase pitch, coke, or aderivative thereof.
 32. The rechargeable alkali metal-selenium cell ofclaim 20 wherein said porous graphitic structure has a specific surfacearea greater than 500 m²/g.
 33. The rechargeable alkali metal-seleniumcell of claim 20 wherein said porous graphitic structure has a specificsurface area greater than 750 m²/g
 34. The rechargeable alkalimetal-selenium cell of claim 20 wherein said cathode has an activematerial utilization rate no less than 80%.
 35. The rechargeable alkalimetal-selenium cell of claim 20 wherein said cathode has an activematerial utilization rate no less than 90%
 36. The rechargeable alkalimetal-selenium cell of claim 20, wherein said cathode contains at least95% by weight of selenium based on the total weight of said porousgraphitic structure and selenium combined.